feat(tracks): add language_priority to sort preferred languages first

subtitle.language_priority and audio.language_priority prepend to the CLI
languages, so the named languages sort first and nothing is filtered.
--exact-lang now applies to the sorts too, through matching_languages, which
follows the same RFC 4647 rule as Tracks.by_language and ignores invalid tags.

docs/guide/cli-reference.md

@@ -97,9 +97,15 @@ unshackle dl [OPTIONS] SERVICE [SERVICE ARGS...]
 | `--require-subs` | - | Required subtitle langs; keeps **all** subs only if these exist. **Cannot combine with `--s-lang`.** |
 | `-fs`, `--forced-subs` | off | Include forced subtitle tracks. |
 | `-fsl`, `--forced-s-lang` | none | Language(s) wanted for forced subtitles; implies `-fs`. |
-| `--exact-lang` | off | Exact matching only: `-l es-419` matches `es-419`, not `es-ES`. |
+| `--exact-lang` | off | Exact matching only: `-l es-419` matches `es-419`, not `es-ES`. Applies to selection and to sort order. |
 | `--sub-format` | - | Output subtitle format (`SRT`/`srt`, `VTT`/`webvtt`, `ASS`/`ssa`, `TTML`, `SMI`, ...), or `original` to keep the source format. |
 
+!!! tip "These flags select languages, they do not order them"
+    Naming languages with `-l` or `-sl` removes the others. To keep every language but put
+    your preferred ones first, use `audio.language_priority` and
+    [`subtitle.language_priority`](../reference/configuration.md#subtitle) in your
+    configuration file.
+
 The special language tokens `orig`, `all`, and `best` are honoured everywhere a language is expected.
 
 ### Title & episode selection

docs/guide/downloading.md

@@ -254,6 +254,26 @@ it is used. You can override each stream type independently:
 unshackle dl -al en EXAMPLE 81234567
 ```
 
+### Sort order
+
+`-l` and `-sl` **select** languages: naming some removes the rest. To keep every language
+but decide which ones come first, set a priority list in your configuration file:
+
+```yaml title="unshackle.yaml"
+audio:
+  language_priority: [orig, en]
+subtitle:
+  group_by: language
+  language_priority: [en, es, fr]
+```
+
+Subtitles then start with English, Spanish, and French. The languages you leave out follow
+alphabetically, after the title's original language. Nothing is removed. Audio works the
+same way, but the languages you leave out keep their bitrate and codec order instead of an
+alphabetical one.
+
+See [`subtitle.language_priority`](../reference/configuration.md#subtitle) for the details.
+
 ### Exact vs fuzzy matching
 
 By default language matching is fuzzy: `-l en` also accepts `en-US`, `en-GB`, etc. (up

docs/guide/subtitles.md

@@ -376,6 +376,8 @@ subtitle:
 | `convert_before_strip` | `true` | `true` / `false` | Convert a subtitle to SRT before stripping SDH (for non-SubtitleEdit engines). |
 | `output_mode` | `mux` | `mux`, `sidecar`, `both` | Whether subtitles are embedded, written as sidecars, or both. |
 | `sidecar_format` | `srt` | a format name (e.g. `srt`, `vtt`) or `original` | Format used for sidecar files. |
+| `group_by` | `type` | `type`, `language` | Whether all tracks of one type stay together, or each language stays next to its own variants. |
+| `language_priority` | *(unset)* | a list of languages | Languages to sort to the top. The rest keep the default order. Nothing is removed. |
 
 !!! tip "SubtitleEdit is optional but recommended"
     Several high-fidelity paths (the best-quality conversions and the SubtitleEdit SDH