Do not waist your time. This is hardcoded ( :-( yes I know, hard to believe) and Non-English translation is absolutely not a priority for ages. All translation in many languages (for both client and server) are ready for very long time but devs have to remove hardcoded strings to makes it happen. This request is pending for... years...
So, my advice : just use it like this. It may change, one day or another. If you can't wait, you may try XBMC : not as good as Plex on some specific parts but finally it's working very well and it is fully translated and evolving very quickly...
