But starting translation with FFXIII would me the most logical thing to do since it's the first in the series and there will be some terms originating from it. Then would be FFXIII-2 with the terms it introduces and then one should finish with Lightning Returns.

FFXIII has the most lines in the Datalog, there aren't THAT many lines in conversations to translate.

FFXIII wouldn't be that hard to translate since it's a linear game. I can help with translating it (Yep, I'm Polish too ;) ). I would actually be really happy to help, just don't count on me when it comes to technicalities since it's my weak point.