Allow specifying source language when translating

Not terrible useful, but it helps with short songs sometimes.

substudy/src/main.rs

@@ -156,6 +156,11 @@ If you have no related text at all, you can omit both `--related-text` and
         /// Path to the subtitle file to translate.
         foreign_subs: PathBuf,
 
+        /// Language to translate from. This can normally be omitted, but it
+        /// might help with closely-related languages or mixed-language content.
+        #[arg(long)]
+        foreign_lang: Option<String>,
+
         /// Target language code (e.g. "en" for English).
         #[arg(long)]
         native_lang: String,
@@ -318,8 +323,12 @@ async fn main() -> Result<()> {
         }
         Args::Translate {
             foreign_subs,
+            foreign_lang,
             native_lang,
-        } => cmd_translate(&ui, &foreign_subs, &native_lang).await,
+        } => {
+            cmd_translate(&ui, &foreign_subs, foreign_lang.as_deref(), &native_lang)
+                .await
+        }
     }
 }
 
@@ -444,10 +453,17 @@ async fn cmd_transcribe(
     Ok(())
 }
 
-async fn cmd_translate(ui: &Ui, foreign_subs: &Path, native_lang: &str) -> Result<()> {
+async fn cmd_translate(
+    ui: &Ui,
+    foreign_subs: &Path,
+    foreign_lang: Option<&str>,
+    native_lang: &str,
+) -> Result<()> {
     let file = SubtitleFile::cleaned_from_path(foreign_subs)?;
+    let foreign_lang = foreign_lang.map(|f| Lang::iso639(f)).transpose()?;
     let native_lang = Lang::iso639(native_lang)?;
-    let translated = translate_subtitle_file(ui, &file, native_lang).await?;
+    let translated =
+        translate_subtitle_file(ui, &file, foreign_lang, native_lang).await?;
     print!("{}", translated.to_string());
     Ok(())
 }

substudy/src/services/oai/translate.rs

@@ -68,12 +68,15 @@ lazy_static! {
 pub async fn translate_subtitle_file(
     ui: &Ui,
     file: &SubtitleFile,
+    from_lang: Option<Lang>,
     to_lang: Lang,
 ) -> Result<SubtitleFile> {
     // Infer the language of the subtitle file.
-    let from_lang = file.detect_language().ok_or_else(|| {
-        anyhow!("Could not detect the language of the input subtitle file")
-    })?;
+    let from_lang = from_lang
+        .or_else(|| file.detect_language())
+        .ok_or_else(|| {
+            anyhow!("Could not detect the language of the input subtitle file")
+        })?;
 
     // Split into chunks of at least `MIN_CHUNK_SIZE`, but then try to end on a
     // sentence boundary. Even if we can't find a sentence boundary, end
@@ -302,7 +305,7 @@ mod tests {
 
         let ui = Ui::init_for_tests();
         let translated =
-            translate_subtitle_file(&ui, &file, Lang::iso639("es").unwrap())
+            translate_subtitle_file(&ui, &file, None, Lang::iso639("es").unwrap())
                 .await
                 .unwrap();
         assert_eq!(translated.subtitles.len(), file.subtitles.len());