opensubtitles: Don't error if the language is unknown

If the current locale is not listed in our list of
OpenSubtitles-supported languages, don't throw an error, but fallback to
selecting English as the language.

https://bugzilla.gnome.org/show_bug.cgi?id=729974
opensubtitles: Add support for Basque

https://bugzilla.gnome.org/show_bug.cgi?id=729974