Add support for LanguageBCP47

src/matroska/ebml.ts

@@ -88,6 +88,7 @@ export enum EBMLId {
 	FlagLacing = 0x9c,
 	Name = 0x536e,
 	Language = 0x22b59c,
+	LanguageBCP47 = 0x22b59d,
 	CodecID = 0x86,
 	CodecPrivate = 0x63a2,
 	CodecDelay = 0x56aa,

src/matroska/matroska-demuxer.ts

@@ -1014,6 +1014,7 @@ export class MatroskaDemuxer extends Demuxer {
 
 			case EBMLId.Language: {
 				if (!this.currentTrack) break;
+				if (this.currentTrack.languageCode) break; // LanguageBCP47 was present, which takes precedence
 
 				this.currentTrack.languageCode = reader.readAsciiString(size);
 
@@ -1022,6 +1023,23 @@ export class MatroskaDemuxer extends Demuxer {
 				}
 			}; break;
 
+			case EBMLId.LanguageBCP47: {
+				if (!this.currentTrack) break;
+
+				const bcp47 = reader.readAsciiString(size);
+				const languageSubtag = bcp47.split('-')[0];
+
+				if (languageSubtag) {
+					// Technically invalid, for now: The language subtag might be a language code from ISO 639-1,
+					// ISO 639-2, ISO 639-3, ISO 639-5 or some other thing (source: Wikipedia). But, `languageCode` is
+					// documented as ISO 639-2. Changing the definition would be a breaking change. This will get
+					// cleaned up in the future by defining languageCode to be BCP 47 instead.
+					this.currentTrack.languageCode = languageSubtag;
+				} else {
+					this.currentTrack.languageCode = UNDETERMINED_LANGUAGE;
+				}
+			}; break;
+
 			case EBMLId.Video: {
 				if (this.currentTrack?.info?.type !== 'video') break;