[extractor/youtube] Support changing extraction language (yt-dlp#4470)

Adds `--extractor-args youtube:lang=<supported lang code>` extractor arg to prefer translated fields (e.g. title and description) of that language, if available, for all YouTube extractors. See README or error message for list of supported language codes.

Closes yt-dlp#387

Authored by: coletdjnz

README.md

@@ -1705,6 +1705,8 @@ The following extractors use this feature:
     * E.g. `all,all,1000,10` will get a maximum of 1000 replies total, with up to 10 replies per thread. `1000,all,100` will get a maximum of 1000 comments, with a maximum of 100 replies total
 * `innertube_host`: Innertube API host to use for all API requests; e.g. `studio.youtube.com`, `youtubei.googleapis.com`. Note that cookies exported from one subdomain will not work on others
 * `innertube_key`: Innertube API key to use for all API requests
+* `lang`: Supported content language code to prefer translated metadata of this language (case-sensitive). By default, video primary language metadata is preferred, with a fallback to `en` translated.
+  * See youtube.py for list of supported content language codes.
 
 #### youtubetab (YouTube playlists, channels, feeds, etc.)
 * `skip`: One or more of `webpage` (skip initial webpage download), `authcheck` (allow the download of playlists requiring authentication when no initial webpage is downloaded. This may cause unwanted behavior, see [#1122](https://github.com/yt-dlp/yt-dlp/pull/1122) for more details)