Elwro contacted CDProjekts Tom Ohle for clarification on the subject (whether TW2 is being written in english) and received some kind of confirmation:

<p style="margin-left:50px;border-style:solid;border-width:1px;border-top-color:#ffffff;padding:5px;border-right-color:#bbbbbb;border-left-color:#ffffff;border-bottom-color:#bbbbbb;">In short, yes that's correct. We have a native English speaker on the team,
so while the game is being written by Polish writer(s) it's being
implemented in the game in English.

So, it is written - originally - in Polish I presume, but then they'll *implement* it in English.
I'm uncertain what's the difference between *implementing in English* and *translating to English* but I'm sure there has to be a difference.