Now when you create an account on Plex website it asks for language and subtitles preferences. It would be great then that it be used also for:
→ get interface of player I use (such as PMP or web interface or any player able to switch language of interface dynamically) automatically use the language based on my setting of language.
→ when I display the details of a media (description, genre…) it should be displayed in that language also (if avalaible from metadata agent used for the media).
Best would be to add a setting for interface language preference in account settings 
Vincèn