Add new language WebAdmin (gl)

Hi Contributors and devs

I’ve been working for the last several weeks translating YunoHost components (three of them at 100%) to my language (galician, language code “gl”).

Log in form had been updated very soon (thanks!) but webadmin config has not galician option available for selection yet when other languages with much less translated text chains are available for selection.

As selecting another language as fallback would prevent issues, would you please add galician to the list? thank you. Doing so I could better understand some context (and use my server in my language, of course).

Please let me know if I shoulb better open an issue on github for this matter.