debug: Added Session Language code as a debug message
debug: Added Session Language code as a debug message

It will show this message when Localization is called by muxed hls or 
dash streams.

```
$ streamlink ... --locale ru_RU
...
[plugin.dash][debug] Parsing MPD URL: ...
[utils.l10n][debug] Language code: ru_RU
```

or your local system code

```
$ streamlink ...
...
[plugin.dash][debug] Parsing MPD URL: ...
[utils.l10n][debug] Language code: xx_XX
```

chhe/streamlink,melmorabity/streamlink,beardypig/streamlink,back-to/streamlink,chhe/streamlink,back-to/streamlink,wlerin/streamlink,gravyboat/streamlink,melmorabity/streamlink,streamlink/streamlink,wlerin/streamlink,streamlink/streamlink,bastimeyer/streamlink,bastimeyer/streamlink,beardypig/streamlink,gravyboat/streamlink