Solved (though in a pricey way)! Replying here in case anyone has the same problem.
I caved in and bought boxed european edition.. and it works! Right now I have two Sims 4 games added to my Origin account, which looks silly. One is Origin's Polish Digital Deluxe and the second one is Multi-language regular one. And I basically have the game in English and I still have the deluxe stuff from Polish edition. Funny thing is - the european edition was even cheaper than Origin's regular, region-locked version. So I see no reasoning behind it (like I would if it was a cut version, but cheaper, for the poorer market to keep westerners from getting too low priced keys on Ebay).
To sum it up - it seems Origin digital keys will become region locked when activated, while a key from a boxed version allows you to install in the language version it came from. In my case - it was all languages available.
Still - it would be much better if EA would give a full info on how to legally buy the language version you desire.