feat: Enhance Audio Language Switching (shaka-project#5223)

Resolves shaka-project#5222.

It also opens the way for improving the compliance with [RFC
5646](https://datatracker.ietf.org/doc/html/rfc5646) in the future.

AUTHORS

@@ -50,6 +50,7 @@ Jozef Chúťka <jozefchutka@gmail.com>
 Jun Hong Chong <chongjunhong@gmail.com>
 Jürgen Kartnaller <kartnaller@lovelysystems.com>
 JW Player <*@jwplayer.com>
+Konstantin Grushetsky <github@grushetsky.net>
 Lucas Gabriel Sánchez <unkiwii@gmail.com>
 Martin Stark <martin.stark@eyevinn.se>
 Matthias Van Parijs <matvp91@gmail.com>

CONTRIBUTORS

@@ -76,6 +76,7 @@ Jozef Chúťka <jozefchutka@gmail.com>
 Julian Domingo <juliandomingo@google.com>
 Jun Hong Chong <chongjunhong@gmail.com>
 Jürgen Kartnaller <kartnaller@lovelysystems.com>
+Konstantin Grushetsky <github@grushetsky.net>
 Leandro Ribeiro Moreira <leandro.ribeiro.moreira@gmail.com>
 Lucas Gabriel Sánchez <unkiwii@gmail.com>
 Martin Stark <martin.stark@eyevinn.se>

lib/util/language_utils.js

@@ -191,12 +191,16 @@ shaka.util.LanguageUtils = class {
   static normalize(locale) {
     const LanguageUtils = shaka.util.LanguageUtils;
 
-    const components = locale.split('-');
+    const privateusePrefix = 'x-';
+    const components = locale.split(`-${privateusePrefix}`);
+    const languageAndRegion = components[0].split('-');
 
-    // We are only going to use the language and the region. If there was
-    // a dialect or anything else, we are throwing it a way.
-    let language = components[0] || '';
-    let region = components[1] || '';
+    // We are only going to use the language, the region and the "privateuse" part (as per https://datatracker.ietf.org/doc/html/rfc5646).
+    // Anything else is thrown away.
+    let language = languageAndRegion[0] || '';
+    let region = languageAndRegion[1] || '';
+    const privateuse = components[1] ?
+        `${privateusePrefix}${components[1]}` : '';
 
     // Convert the language to lower case. It is standard for the language code
     // to be in lower case, but it will also make the map look-up easier.
@@ -208,9 +212,8 @@ shaka.util.LanguageUtils = class {
     // and this will be a no-op.
     region = region.toUpperCase();
 
-    return region ?
-           language + '-' + region :
-           language;
+    return `${region ? `${language}-${region}` : language}${
+        privateuse ? `-${privateuse}` : ''}`;
   }
 
   /**

test/util/language_utils_unit.js

@@ -111,16 +111,23 @@ describe('LanguageUtils', () => {
       expect(LanguageUtils.normalize('EN')).toBe('en');
     });
 
-    it('standardized region', () => {
+    it('standardizes region', () => {
       expect(LanguageUtils.normalize('en-US')).toBe('en-US');
       expect(LanguageUtils.normalize('en-us')).toBe('en-US');
     });
 
-    it('ignored unknown base languages', () => {
+    it('ignores unknown base languages', () => {
       expect(LanguageUtils.normalize('elvish')).toBe('elvish');
       expect(LanguageUtils.normalize(
           'elvish-woodland')).toBe('elvish-WOODLAND');
     });
+
+    it('supports private use tag', () => {
+      expect(LanguageUtils.normalize('eng-us-x-1')).toBe('en-US-x-1');
+      expect(LanguageUtils.normalize('eng-x-1')).toBe('en-x-1');
+      expect(LanguageUtils.normalize(
+          'elvish-woodland-x-1')).toBe('elvish-WOODLAND-x-1');
+    });
   });
 
   describe('getLocaleForText', () => {