Match and mux in dropped external subtitles

Drag in subtitle files alongside .mkv files and they're matched to
the right video by episode code (S04E06, zero-padding and separator
insensitive, plus 4x06 as a fallback) and muxed in as extra subtitle
tracks. Any subtitle format mkvmerge accepts is recognized: SRT,
(S)SA, VobSub, WebVTT, PGS/SUP, USF, SAMI, TTML/DFXP, STL, Kate.

Matching works regardless of drop order — subtitles dropped before
their video are held pending until a matching video shows up, and
vice versa. Language, forced, and SDH flags are guessed from common
filename tokens (e.g. "eng", "forced", "sdh") to set the muxed
track's language/name/flags; unmatched subtitles are reported in the
status bar.

Futonizer.csproj

@@ -9,8 +9,8 @@
     <ImplicitUsings>enable</ImplicitUsings>
     <AssemblyName>Futonizer</AssemblyName>
     <RootNamespace>Futonizer</RootNamespace>
-    <Version>1.0.5</Version>
+    <Version>1.0.6</Version>
-    <FileVersion>1.0.5.0</FileVersion>
+    <FileVersion>1.0.6.0</FileVersion>
     <ApplicationIcon>Assets\app.ico</ApplicationIcon>
   </PropertyGroup>
 

MainWindow.xaml

@@ -137,7 +137,7 @@
                         <Grid Grid.Row="1">
                             <TextBlock x:Name="DropHint"
                                        HorizontalAlignment="Center" VerticalAlignment="Center"
-                                       Text="Drop .mkv files&#10;onto this window"
+                                       Text="Drop .mkv files (and subtitles)&#10;onto this window"
                                        Opacity="0.4" FontSize="14" TextAlignment="Center"
                                        IsHitTestVisible="False"/>
 
@@ -158,6 +158,7 @@
                                                 <ColumnDefinition Width="16"/>
                                                 <ColumnDefinition Width="*"/>
                                                 <ColumnDefinition Width="Auto"/>
+                                                <ColumnDefinition Width="Auto"/>
                                                 <ColumnDefinition Width="18"/>
                                             </Grid.ColumnDefinitions>
 
@@ -179,12 +180,21 @@
                                                        FontSize="12" VerticalAlignment="Center" Margin="6,0"
                                                        TextTrimming="CharacterEllipsis"/>
 
-                                            <TextBlock Grid.Column="2" Text="{Binding FileSizeDisplay}"
+                                            <Border Grid.Column="2" CornerRadius="8" Padding="5,1" Margin="2,0"
+                                                    VerticalAlignment="Center"
+                                                    Background="{DynamicResource AccentFillColorSecondaryBrush}"
+                                                    Visibility="{Binding HasExternalSubtitles, Converter={StaticResource BoolToVis}}"
+                                                    ToolTip="{Binding ExternalSubtitlesTooltip}">
+                                                <TextBlock Text="{Binding ExternalSubtitles.Count, StringFormat={}+{0} sub}"
+                                                           FontSize="9" FontWeight="SemiBold"/>
+                                            </Border>
+
+                                            <TextBlock Grid.Column="3" Text="{Binding FileSizeDisplay}"
                                                        FontSize="11" VerticalAlignment="Center" Margin="4,0"
                                                        Opacity="0.6"
                                                        Foreground="{DynamicResource TextFillColorSecondaryBrush}"/>
 
-                                            <Button Grid.Column="3" Content="&#xD7;"
+                                            <Button Grid.Column="4" Content="&#xD7;"
                                                     Tag="{Binding}"
                                                     Click="RemoveFile_Click"
                                                     Background="Transparent" BorderThickness="0"
@@ -209,6 +219,55 @@
                         BorderBrush="{DynamicResource ControlElevationBorderBrush}"
                         BorderThickness="1">
                     <Grid>
+                        <Grid.RowDefinitions>
+                            <RowDefinition Height="Auto"/>
+                            <RowDefinition Height="*"/>
+                        </Grid.RowDefinitions>
+
+                        <!-- External subtitles matched to the selected file -->
+                        <Border x:Name="ExternalSubsPanel" Grid.Row="0" Margin="10,8,10,0"
+                                Padding="8,6" CornerRadius="6" Visibility="Collapsed"
+                                Background="{DynamicResource ControlFillColorSecondaryBrush}">
+                            <StackPanel>
+                                <TextBlock Text="External subtitles to mux in" FontSize="11"
+                                           FontWeight="SemiBold" Opacity="0.7" Margin="0,0,0,4"/>
+                                <ItemsControl x:Name="ExternalSubsList">
+                                    <ItemsControl.ItemsPanel>
+                                        <ItemsPanelTemplate>
+                                            <WrapPanel Orientation="Horizontal"/>
+                                        </ItemsPanelTemplate>
+                                    </ItemsControl.ItemsPanel>
+                                    <ItemsControl.ItemTemplate>
+                                        <DataTemplate>
+                                            <Border Background="{DynamicResource ControlFillColorDefaultBrush}"
+                                                    BorderBrush="{DynamicResource ControlElevationBorderBrush}"
+                                                    BorderThickness="1" CornerRadius="4"
+                                                    Margin="0,0,6,6" Padding="6,3">
+                                                <StackPanel Orientation="Horizontal">
+                                                    <TextBlock Text="{Binding LanguageDisplay}" FontSize="11"
+                                                               FontWeight="SemiBold" VerticalAlignment="Center"/>
+                                                    <TextBlock Text="{Binding FileName}" FontSize="11" Opacity="0.6"
+                                                               Margin="6,0,0,0" VerticalAlignment="Center"
+                                                               MaxWidth="220" TextTrimming="CharacterEllipsis"/>
+                                                    <TextBlock Text="DEFAULT" FontSize="9" FontWeight="Bold"
+                                                               Foreground="{DynamicResource AccentTextFillColorPrimaryBrush}"
+                                                               Margin="6,0,0,0" VerticalAlignment="Center"
+                                                               Visibility="{Binding IsDefault, Converter={StaticResource BoolToVis}}"/>
+                                                    <Button Content="&#xD7;" Tag="{Binding}"
+                                                            Click="RemoveExternalSubtitle_Click"
+                                                            Background="Transparent" BorderThickness="0"
+                                                            Padding="6,0,0,0" Cursor="Hand" FontSize="13"
+                                                            VerticalAlignment="Center"
+                                                            Foreground="{DynamicResource TextFillColorSecondaryBrush}"/>
+                                                </StackPanel>
+                                            </Border>
+                                        </DataTemplate>
+                                    </ItemsControl.ItemTemplate>
+                                </ItemsControl>
+                            </StackPanel>
+                        </Border>
+
+                        <Grid Grid.Row="1">
                         <TextBlock x:Name="NoSelectionHint"
                                    HorizontalAlignment="Center" VerticalAlignment="Center"
                                    Text="Select a file to view its tracks"
@@ -328,6 +387,7 @@
                                 </DataGridTemplateColumn>
                             </DataGrid.Columns>
                         </DataGrid>
+                        </Grid>
                     </Grid>
                 </Border>
 

MainWindow.xaml.cs

@@ -20,6 +20,7 @@ public partial class MainWindow : FluentWindow
 
     private AppSettings _settings = new();
     private readonly ObservableCollection<QueuedFileItem> _fileQueue = new();
+    private readonly List<string> _pendingSubtitles = new();
     private readonly SemaphoreSlim _scanSemaphore = new(MaxConcurrentScans);
     private bool _isProcessing;
     private bool _propagatingSelection;
@@ -196,10 +197,9 @@ public partial class MainWindow : FluentWindow
         if (e.Data.GetDataPresent(DataFormats.FileDrop))
         {
             var paths = (string[])e.Data.GetData(DataFormats.FileDrop);
-            bool hasMkv = paths.Any(p =>
+            bool hasAcceptedFile = paths.Any(p =>
-                string.Equals(Path.GetExtension(p), ".mkv", StringComparison.OrdinalIgnoreCase)
+                File.Exists(p) && (IsMkvFile(p) || SubtitleMatcher.IsSubtitleFile(p)));
-                && File.Exists(p));
+            e.Effects = hasAcceptedFile ? DragDropEffects.Copy : DragDropEffects.None;
-            e.Effects = hasMkv ? DragDropEffects.Copy : DragDropEffects.None;
         }
         else
         {
@@ -208,6 +208,9 @@ public partial class MainWindow : FluentWindow
         e.Handled = true;
     }
 
+    private static bool IsMkvFile(string path)
+        => string.Equals(Path.GetExtension(path), ".mkv", StringComparison.OrdinalIgnoreCase);
+
     private void Window_Drop(object sender, DragEventArgs e)
     {
         if (!e.Data.GetDataPresent(DataFormats.FileDrop)) return;
@@ -215,28 +218,158 @@ public partial class MainWindow : FluentWindow
         Activate();
 
         var paths = (string[])e.Data.GetData(DataFormats.FileDrop);
-        var newFiles = paths
+
-            .Where(p => string.Equals(Path.GetExtension(p), ".mkv", StringComparison.OrdinalIgnoreCase)
+        var newVideoPaths = paths
+            .Where(p => IsMkvFile(p)
                         && File.Exists(p)
                         && !_fileQueue.Any(q => string.Equals(q.FilePath, p, StringComparison.OrdinalIgnoreCase)))
             .ToList();
 
-        foreach (var path in newFiles)
+        var subtitlePaths = paths
+            .Where(p => SubtitleMatcher.IsSubtitleFile(p) && File.Exists(p))
+            .ToList();
+
+        var newItems = new List<QueuedFileItem>();
+        foreach (var path in newVideoPaths)
         {
             var item = new QueuedFileItem(path);
             item.PropertyChanged += QueuedFileItem_PropertyChanged;
             _fileQueue.Add(item);
+            newItems.Add(item);
             _ = ScanFileItemAsync(item);
         }
 
+        // Newly-added videos might satisfy subtitles dropped in earlier (or
+        // in this same batch, if the subtitle sorts before its video).
+        foreach (var item in newItems)
+            MatchPendingSubtitlesToFile(item);
+
+        int matchedSubtitles = 0;
+        foreach (var subPath in subtitlePaths)
+        {
+            if (MatchSubtitleToQueue(subPath))
+                matchedSubtitles++;
+            else if (!_pendingSubtitles.Contains(subPath, StringComparer.OrdinalIgnoreCase))
+                _pendingSubtitles.Add(subPath);
+        }
+
         UpdateLoadingOverlay();
 
-        if (newFiles.Count > 0)
+        if (newItems.Count > 0)
         {
             var first = _fileQueue.FirstOrDefault(q =>
-                string.Equals(q.FilePath, newFiles[0], StringComparison.OrdinalIgnoreCase));
+                string.Equals(q.FilePath, newVideoPaths[0], StringComparison.OrdinalIgnoreCase));
             if (first != null) FileQueueList.SelectedItem = first;
         }
+
+        int unmatchedSubtitles = subtitlePaths.Count - matchedSubtitles;
+        if (matchedSubtitles > 0 || unmatchedSubtitles > 0)
+        {
+            StatusText.Text = unmatchedSubtitles > 0
+                ? $"Matched {matchedSubtitles} subtitle(s); {unmatchedSubtitles} couldn't be matched to an episode code (S04E06 style)."
+                : $"Matched {matchedSubtitles} subtitle(s) to their episode.";
+        }
+    }
+
+    // ── Subtitle matching ───────────────────────────────────────────────────
+
+    /// <summary>
+    /// Attempts to attach a dropped subtitle file to whichever queued video(s)
+    /// share the same episode code (e.g. S04E06). Falls back to the single
+    /// queued file when there's exactly one and no episode code could be
+    /// extracted from the subtitle's filename. Returns false if no target
+    /// could be determined, in which case the file is kept pending until a
+    /// matching video is dropped later.
+    /// </summary>
+    private bool MatchSubtitleToQueue(string subtitlePath)
+    {
+        string subFileName = Path.GetFileName(subtitlePath);
+        string? code = SubtitleMatcher.ExtractEpisodeCode(subFileName);
+
+        List<QueuedFileItem> targets;
+        if (code != null)
+        {
+            targets = _fileQueue
+                .Where(q => string.Equals(SubtitleMatcher.ExtractEpisodeCode(q.FileName), code, StringComparison.OrdinalIgnoreCase))
+                .ToList();
+        }
+        else if (_fileQueue.Count == 1)
+        {
+            targets = new List<QueuedFileItem> { _fileQueue[0] };
+        }
+        else
+        {
+            targets = new List<QueuedFileItem>();
+        }
+
+        if (targets.Count == 0) return false;
+
+        foreach (var target in targets)
+            AttachExternalSubtitle(target, subtitlePath);
+
+        return true;
+    }
+
+    /// <summary>
+    /// Called whenever a new video is added to the queue: checks the pending
+    /// (previously unmatched) subtitle files for one sharing this video's
+    /// episode code, and attaches + removes any matches from the pending list.
+    /// </summary>
+    private void MatchPendingSubtitlesToFile(QueuedFileItem item)
+    {
+        if (_pendingSubtitles.Count == 0) return;
+
+        string? code = SubtitleMatcher.ExtractEpisodeCode(item.FileName);
+        if (code == null) return;
+
+        var matches = _pendingSubtitles
+            .Where(p => string.Equals(SubtitleMatcher.ExtractEpisodeCode(Path.GetFileName(p)), code, StringComparison.OrdinalIgnoreCase))
+            .ToList();
+
+        foreach (var match in matches)
+        {
+            AttachExternalSubtitle(item, match);
+            _pendingSubtitles.Remove(match);
+        }
+    }
+
+    private static void AttachExternalSubtitle(QueuedFileItem target, string subtitlePath)
+    {
+        if (target.ExternalSubtitles.Any(s => string.Equals(s.FilePath, subtitlePath, StringComparison.OrdinalIgnoreCase)))
+            return;
+
+        var info = SubtitleLanguageHelper.Detect(Path.GetFileName(subtitlePath));
+        var sub = new ExternalSubtitleTrack(subtitlePath)
+        {
+            Language = info.Language,
+            TrackName = info.TrackName,
+            Forced = info.Forced,
+            HearingImpaired = info.HearingImpaired,
+            IsDefault = target.ExternalSubtitles.Count == 0,
+        };
+        target.ExternalSubtitles.Add(sub);
+    }
+
+    private void RemoveExternalSubtitle_Click(object sender, RoutedEventArgs e)
+    {
+        if (sender is not System.Windows.Controls.Button btn || btn.Tag is not ExternalSubtitleTrack sub) return;
+
+        var owner = _fileQueue.FirstOrDefault(f => f.ExternalSubtitles.Contains(sub));
+        if (owner == null) return;
+
+        bool wasDefault = sub.IsDefault;
+        owner.ExternalSubtitles.Remove(sub);
+
+        if (wasDefault)
+        {
+            var next = owner.ExternalSubtitles.FirstOrDefault();
+            if (next != null) next.IsDefault = true;
+        }
+
+        if (ReferenceEquals(FileQueueList.SelectedItem, owner))
+            ExternalSubsPanel.Visibility = owner.HasExternalSubtitles ? Visibility.Visible : Visibility.Collapsed;
+
+        e.Handled = true;
     }
 
     // ── File scanning ─────────────────────────────────────────────────────────
@@ -457,12 +590,16 @@ public partial class MainWindow : FluentWindow
             TrackGrid.ItemsSource = file.Tracks;
             TrackGrid.Visibility = Visibility.Visible;
             NoSelectionHint.Visibility = Visibility.Collapsed;
+            ExternalSubsList.ItemsSource = file.ExternalSubtitles;
+            ExternalSubsPanel.Visibility = file.HasExternalSubtitles ? Visibility.Visible : Visibility.Collapsed;
         }
         else
         {
             TrackGrid.ItemsSource = null;
             TrackGrid.Visibility = Visibility.Collapsed;
             NoSelectionHint.Visibility = Visibility.Visible;
+            ExternalSubsList.ItemsSource = null;
+            ExternalSubsPanel.Visibility = Visibility.Collapsed;
         }
     }
 

Models/ExternalSubtitleTrack.cs

@@ -0,0 +1,58 @@
+using System.ComponentModel;
+using System.IO;
+using System.Runtime.CompilerServices;
+
+namespace Futonizer.Models;
+
+/// <summary>
+/// Represents one external subtitle file that was drag-and-dropped in and
+/// matched to a <see cref="QueuedFileItem"/> by episode code (e.g. S04E06),
+/// to be muxed in as an additional subtitle track when the file is processed.
+/// </summary>
+public class ExternalSubtitleTrack : INotifyPropertyChanged
+{
+    private bool _isDefault;
+
+    public string FilePath { get; }
+    public string FileName => Path.GetFileName(FilePath);
+
+    /// <summary>Uppercased extension without the dot, e.g. "SRT", "ASS", "SUP".</summary>
+    public string FormatDisplay => Path.GetExtension(FilePath).TrimStart('.').ToUpperInvariant();
+
+    /// <summary>ISO 639-2 language code detected from the filename, or "und" if unknown.</summary>
+    public string Language { get; init; } = "und";
+
+    /// <summary>Track name to embed in the output file; may be empty.</summary>
+    public string TrackName { get; init; } = string.Empty;
+
+    public bool Forced { get; init; }
+    public bool HearingImpaired { get; init; }
+
+    /// <summary>
+    /// Whether this is the subtitle track flagged default in the output.
+    /// Exactly one external subtitle per file should have this set at a time.
+    /// </summary>
+    public bool IsDefault
+    {
+        get => _isDefault;
+        set
+        {
+            if (_isDefault == value) return;
+            _isDefault = value;
+            OnPropertyChanged();
+        }
+    }
+
+    /// <summary>Short "FORMAT · LANG" label shown as a chip in the UI.</summary>
+    public string LanguageDisplay => $"{FormatDisplay} · {(string.Equals(Language, "und", StringComparison.OrdinalIgnoreCase) ? "UND" : Language.ToUpperInvariant())}";
+
+    public ExternalSubtitleTrack(string filePath)
+    {
+        FilePath = filePath;
+    }
+
+    public event PropertyChangedEventHandler? PropertyChanged;
+
+    private void OnPropertyChanged([CallerMemberName] string? name = null)
+        => PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(name));
+}

Models/QueuedFileItem.cs

@@ -43,6 +43,19 @@ public class QueuedFileItem : INotifyPropertyChanged
     public string FileSizeDisplay { get; }
     public ObservableCollection<TrackItem> Tracks { get; } = new();
 
+    /// <summary>
+    /// External subtitle files (dropped separately and matched to this file by
+    /// episode code, e.g. S04E06) that will be muxed in as additional
+    /// subtitle tracks when this file is processed.
+    /// </summary>
+    public ObservableCollection<ExternalSubtitleTrack> ExternalSubtitles { get; } = new();
+
+    public bool HasExternalSubtitles => ExternalSubtitles.Count > 0;
+
+    public string ExternalSubtitlesTooltip => ExternalSubtitles.Count == 0
+        ? string.Empty
+        : "External subtitles to mux in:\n" + string.Join("\n", ExternalSubtitles.Select(s => $"- {s.FileName} ({s.LanguageDisplay})"));
+
     public string Status
     {
         get => _status;
@@ -71,7 +84,12 @@ public class QueuedFileItem : INotifyPropertyChanged
             int audioSelected = Tracks.Count(t => t.Type == "audio" && t.Copy);
             int subtitleSelected = Tracks.Count(t => t.Type == "subtitles" && t.Copy);
 
-            return audioSelected == 1 && subtitleSelected == 1
+            // A subtitle requirement is met either by picking exactly one
+            // internal track, or by having at least one matched external
+            // subtitle file (which can also be combined with one internal track).
+            bool subtitleOk = subtitleSelected == 1 || (subtitleSelected == 0 && ExternalSubtitles.Count > 0);
+
+            return audioSelected == 1 && subtitleOk
                 ? FileLoadState.Correct
                 : FileLoadState.Incorrect;
         }
@@ -87,6 +105,7 @@ public class QueuedFileItem : INotifyPropertyChanged
         FilePath = filePath;
         FileSizeDisplay = FormatFileSize(filePath);
         Tracks.CollectionChanged += OnTracksChanged;
+        ExternalSubtitles.CollectionChanged += OnExternalSubtitlesChanged;
     }
 
     private static string FormatFileSize(string filePath)
@@ -112,6 +131,13 @@ public class QueuedFileItem : INotifyPropertyChanged
         }
     }
 
+    private void OnExternalSubtitlesChanged(object? sender, NotifyCollectionChangedEventArgs e)
+    {
+        OnPropertyChanged(nameof(HasExternalSubtitles));
+        OnPropertyChanged(nameof(ExternalSubtitlesTooltip));
+        NotifyLoadState();
+    }
+
     private void OnTracksChanged(object? sender, NotifyCollectionChangedEventArgs e)
     {
         if (e.NewItems != null)

Services/MkvService.cs

@@ -258,12 +258,13 @@ public class MkvService
         IReadOnlyList<int> videoKeepIds,
         IReadOnlyList<int> audioKeepIds,
         IReadOnlyList<int> subtitleKeepIds,
+        IReadOnlyList<ExternalSubtitleTrack>? externalSubtitles = null,
         IProgress<ProcessingProgress>? progress = null,
         CancellationToken ct = default)
     {
         return StripTracksInternalAsync(
             filePath, Path.GetFileName(filePath), outputFilePath,
-            videoKeepIds, audioKeepIds, subtitleKeepIds,
+            videoKeepIds, audioKeepIds, subtitleKeepIds, externalSubtitles,
             progress, ct);
     }
 
@@ -281,6 +282,7 @@ public class MkvService
             null,                  // null = keep all video
             scan.AudioKeepIds,
             scan.SubtitleKeepIds,
+            null,
             progress, ct);
     }
 
@@ -386,6 +388,7 @@ public class MkvService
         IReadOnlyList<int>? videoKeepIds,
         IReadOnlyList<int>? audioKeepIds,
         IReadOnlyList<int>? subtitleKeepIds,
+        IReadOnlyList<ExternalSubtitleTrack>? externalSubtitles,
         IProgress<ProcessingProgress>? progress,
         CancellationToken ct)
     {
@@ -422,16 +425,52 @@ public class MkvService
             AddTrackTypeArgs(psi, "--audio-tracks", "--no-audio", audioKeepIds);
             AddTrackTypeArgs(psi, "--subtitle-tracks", "--no-subtitles", subtitleKeepIds);
 
-            // The one subtitle track the user chose to keep is always flagged
+            // Only one subtitle track in the whole output should ever be
-            // as the default subtitle track in the output.
+            // flagged default. If any external subtitle is marked default,
+            // it takes priority over the kept internal subtitle track.
+            bool hasExternalDefault = externalSubtitles != null && externalSubtitles.Any(s => s.IsDefault);
+
             if (subtitleKeepIds != null && subtitleKeepIds.Count == 1)
             {
                 psi.ArgumentList.Add("--default-track-flag");
-                psi.ArgumentList.Add($"{subtitleKeepIds[0]}:yes");
+                psi.ArgumentList.Add(hasExternalDefault ? $"{subtitleKeepIds[0]}:no" : $"{subtitleKeepIds[0]}:yes");
             }
 
             psi.ArgumentList.Add(filePath);
 
+            // Append each matched external subtitle file as its own input,
+            // with per-file options (language/name/flags) scoped to it.
+            if (externalSubtitles != null)
+            {
+                foreach (var sub in externalSubtitles)
+                {
+                    if (!string.IsNullOrEmpty(sub.Language) && !string.Equals(sub.Language, "und", StringComparison.OrdinalIgnoreCase))
+                    {
+                        psi.ArgumentList.Add("--language");
+                        psi.ArgumentList.Add($"0:{sub.Language}");
+                    }
+                    if (!string.IsNullOrWhiteSpace(sub.TrackName))
+                    {
+                        psi.ArgumentList.Add("--track-name");
+                        psi.ArgumentList.Add($"0:{sub.TrackName}");
+                    }
+                    if (sub.Forced)
+                    {
+                        psi.ArgumentList.Add("--forced-display-flag");
+                        psi.ArgumentList.Add("0:yes");
+                    }
+                    if (sub.HearingImpaired)
+                    {
+                        psi.ArgumentList.Add("--hearing-impaired-flag");
+                        psi.ArgumentList.Add("0:yes");
+                    }
+                    psi.ArgumentList.Add("--default-track-flag");
+                    psi.ArgumentList.Add(sub.IsDefault ? "0:yes" : "0:no");
+
+                    psi.ArgumentList.Add(sub.FilePath);
+                }
+            }
+
             using var process = new Process { StartInfo = psi };
             process.Start();
 

Services/SubtitleLanguageHelper.cs

@@ -0,0 +1,105 @@
+using System.IO;
+
+namespace Futonizer.Services;
+
+/// <summary>
+/// Result of scanning an external subtitle's filename for language and
+/// display hints.
+/// </summary>
+public readonly record struct SubtitleLanguageInfo(string Language, bool Forced, bool HearingImpaired, string TrackName);
+
+/// <summary>
+/// Best-effort language/flag detection for external subtitle files, based on
+/// common filename tokens (e.g. "Show.S04E06.eng.srt", "Show.S04E06.forced.srt").
+/// Never throws — falls back to an "und" (undetermined) language with no
+/// track name when nothing recognisable is found.
+/// </summary>
+public static class SubtitleLanguageHelper
+{
+    private static readonly Dictionary<string, string> TokenToIso6392 = new(StringComparer.OrdinalIgnoreCase)
+    {
+        ["en"] = "eng", ["eng"] = "eng", ["english"] = "eng",
+        ["ja"] = "jpn", ["jp"] = "jpn", ["jpn"] = "jpn", ["japanese"] = "jpn",
+        ["es"] = "spa", ["spa"] = "spa", ["spanish"] = "spa", ["esp"] = "spa",
+        ["fr"] = "fre", ["fre"] = "fre", ["fra"] = "fre", ["french"] = "fre",
+        ["de"] = "ger", ["ger"] = "ger", ["deu"] = "ger", ["german"] = "ger",
+        ["it"] = "ita", ["ita"] = "ita", ["italian"] = "ita",
+        ["pt"] = "por", ["por"] = "por", ["portuguese"] = "por", ["ptbr"] = "por",
+        ["ru"] = "rus", ["rus"] = "rus", ["russian"] = "rus",
+        ["zh"] = "chi", ["chi"] = "chi", ["zho"] = "chi", ["chinese"] = "chi", ["cmn"] = "chi", ["yue"] = "chi",
+        ["ko"] = "kor", ["kor"] = "kor", ["korean"] = "kor",
+        ["ar"] = "ara", ["ara"] = "ara", ["arabic"] = "ara",
+        ["nl"] = "dut", ["dut"] = "dut", ["nld"] = "dut", ["dutch"] = "dut",
+        ["sv"] = "swe", ["swe"] = "swe", ["swedish"] = "swe",
+        ["no"] = "nor", ["nor"] = "nor", ["norwegian"] = "nor",
+        ["da"] = "dan", ["dan"] = "dan", ["danish"] = "dan",
+        ["fi"] = "fin", ["fin"] = "fin", ["finnish"] = "fin",
+        ["pl"] = "pol", ["pol"] = "pol", ["polish"] = "pol",
+        ["tr"] = "tur", ["tur"] = "tur", ["turkish"] = "tur",
+        ["vi"] = "vie", ["vie"] = "vie", ["vietnamese"] = "vie",
+        ["th"] = "tha", ["tha"] = "tha", ["thai"] = "tha",
+        ["id"] = "ind", ["ind"] = "ind", ["indonesian"] = "ind",
+        ["he"] = "heb", ["heb"] = "heb", ["hebrew"] = "heb",
+        ["hin"] = "hin", ["hindi"] = "hin",
+        ["cs"] = "cze", ["cze"] = "cze", ["ces"] = "cze", ["czech"] = "cze",
+        ["el"] = "gre", ["gre"] = "gre", ["ell"] = "gre", ["greek"] = "gre",
+        ["hu"] = "hun", ["hun"] = "hun", ["hungarian"] = "hun",
+        ["ro"] = "rum", ["rum"] = "rum", ["ron"] = "rum", ["romanian"] = "rum",
+        ["uk"] = "ukr", ["ukr"] = "ukr", ["ukrainian"] = "ukr",
+    };
+
+    private static readonly Dictionary<string, string> Iso6392ToDisplayName = new(StringComparer.OrdinalIgnoreCase)
+    {
+        ["eng"] = "English", ["jpn"] = "Japanese", ["spa"] = "Spanish", ["fre"] = "French",
+        ["ger"] = "German", ["ita"] = "Italian", ["por"] = "Portuguese", ["rus"] = "Russian",
+        ["chi"] = "Chinese", ["kor"] = "Korean", ["ara"] = "Arabic", ["dut"] = "Dutch",
+        ["swe"] = "Swedish", ["nor"] = "Norwegian", ["dan"] = "Danish", ["fin"] = "Finnish",
+        ["pol"] = "Polish", ["tur"] = "Turkish", ["vie"] = "Vietnamese", ["tha"] = "Thai",
+        ["ind"] = "Indonesian", ["heb"] = "Hebrew", ["hin"] = "Hindi", ["cze"] = "Czech",
+        ["gre"] = "Greek", ["hun"] = "Hungarian", ["rum"] = "Romanian", ["ukr"] = "Ukrainian",
+    };
+
+    private static readonly char[] TokenSeparators = { '.', '_', '-', ' ', '(', ')', '[', ']' };
+
+    public static SubtitleLanguageInfo Detect(string fileName)
+    {
+        string stem = Path.GetFileNameWithoutExtension(fileName);
+        var tokens = stem.Split(TokenSeparators, StringSplitOptions.RemoveEmptyEntries);
+
+        string language = "und";
+        bool forced = false;
+        bool hearingImpaired = false;
+
+        foreach (var token in tokens)
+        {
+            if (string.Equals(token, "forced", StringComparison.OrdinalIgnoreCase))
+            {
+                forced = true;
+                continue;
+            }
+            if (string.Equals(token, "sdh", StringComparison.OrdinalIgnoreCase)
+                || string.Equals(token, "cc", StringComparison.OrdinalIgnoreCase))
+            {
+                hearingImpaired = true;
+                continue;
+            }
+            if (language == "und" && TokenToIso6392.TryGetValue(token, out var iso))
+                language = iso;
+        }
+
+        string trackName = string.Empty;
+        if (Iso6392ToDisplayName.TryGetValue(language, out var displayName))
+            trackName = displayName;
+
+        if (forced || hearingImpaired)
+        {
+            var suffixes = new List<string>();
+            if (forced) suffixes.Add("Forced");
+            if (hearingImpaired) suffixes.Add("SDH");
+            string suffix = $" ({string.Join(", ", suffixes)})";
+            trackName = string.IsNullOrEmpty(trackName) ? suffix.Trim(' ', '(', ')') : trackName + suffix;
+        }
+
+        return new SubtitleLanguageInfo(language, forced, hearingImpaired, trackName);
+    }
+}

Services/SubtitleMatcher.cs

@@ -0,0 +1,61 @@
+using System.IO;
+using System.Text.RegularExpressions;
+
+namespace Futonizer.Services;
+
+/// <summary>
+/// Recognises subtitle files by extension and extracts a normalized episode
+/// code (e.g. "S04E06") from a filename, used to match dropped subtitle
+/// files to the video file with the same episode code.
+/// </summary>
+public static class SubtitleMatcher
+{
+    /// <summary>
+    /// Every subtitle container/format extension mkvmerge (and therefore
+    /// Futonizer) can mux in: SubRip, (Advanced) SubStation Alpha, VobSub,
+    /// WebVTT, HDMV/PGS, Universal Subtitle Format, SAMI, TTML/DFXP, EBU-STL,
+    /// MicroDVD/SubViewer (both commonly saved with a plain ".sub" extension),
+    /// and Kate.
+    /// </summary>
+    public static readonly string[] SupportedExtensions =
+    {
+        ".srt", ".ass", ".ssa", ".sub", ".idx", ".vtt", ".webvtt",
+        ".sup", ".pgs", ".usf", ".smi", ".sami", ".ttml", ".dfxp",
+        ".stl", ".kate",
+    };
+
+    public static bool IsSubtitleFile(string path)
+    {
+        string ext = Path.GetExtension(path);
+        return !string.IsNullOrEmpty(ext)
+               && SupportedExtensions.Contains(ext, StringComparer.OrdinalIgnoreCase);
+    }
+
+    // "S04E06", "S4E6", "S04 E06", "S04-E06", "S04.E06" — the standard scene/anime tagging scheme.
+    private static readonly Regex SeasonEpisodeRegex =
+        new(@"S(\d{1,4})[\s._-]*E(\d{1,4})", RegexOptions.IgnoreCase | RegexOptions.Compiled);
+
+    // Fallback: "4x06" style season x episode separator.
+    private static readonly Regex SeasonXEpisodeRegex =
+        new(@"(?<![\d.])(\d{1,2})x(\d{2,3})(?!\d)", RegexOptions.IgnoreCase | RegexOptions.Compiled);
+
+    /// <summary>
+    /// Extracts and normalizes the episode code from a filename (e.g.
+    /// "Show.S4E6.1080p.mkv" and "Show - 04x06 - Title.srt" both return
+    /// "S04E06"), so files can be matched regardless of zero-padding or
+    /// separator style. Returns null if no episode code is found.
+    /// </summary>
+    public static string? ExtractEpisodeCode(string fileName)
+    {
+        var match = SeasonEpisodeRegex.Match(fileName);
+        if (!match.Success)
+            match = SeasonXEpisodeRegex.Match(fileName);
+
+        if (!match.Success) return null;
+
+        if (!int.TryParse(match.Groups[1].Value, out int season)) return null;
+        if (!int.TryParse(match.Groups[2].Value, out int episode)) return null;
+
+        return $"S{season:D2}E{episode:D2}";
+    }
+}

Views/ProgressWindow.xaml.cs

@@ -174,7 +174,7 @@ public partial class ProgressWindow : FluentWindow
                         .Where(t => t.Type == "subtitles" && t.Copy)
                         .Select(t => t.Id).ToList();
 
-                    bool nothingToStrip = file.Tracks.All(t => t.Copy);
+                    bool nothingToStrip = file.Tracks.All(t => t.Copy) && file.ExternalSubtitles.Count == 0;
 
                     var progress = new Progress<ProcessingProgress>(p =>
                     {
@@ -187,10 +187,14 @@ public partial class ProgressWindow : FluentWindow
                         : await service.StripTracksAsync(
                             file.FilePath, outputFilePath,
                             videoIds, audioIds, subIds,
+                            file.ExternalSubtitles.ToList(),
                             progress, ct);
 
                     string tag = result.Success ? "[DONE]" : "[FAIL]";
-                    AppendLog($"{tag} {Path.GetFileName(result.OutputFilePath)} — {result.Message}");
+                    string extInfo = file.ExternalSubtitles.Count > 0
+                        ? $" (+{file.ExternalSubtitles.Count} external sub{(file.ExternalSubtitles.Count == 1 ? "" : "s")})"
+                        : string.Empty;
+                    AppendLog($"{tag} {Path.GetFileName(result.OutputFilePath)}{extInfo} — {result.Message}");
 
                     HistoryService.Add(new HistoryEntry
                     {