Language problem (even after installing language pack and changing registry)

I will try to keep this short;

I bought a product key and when I wanted to start the download, the only language options were Polish, Russian and Czech (I want German but would be ok with English as well).

Wrote an e-mail to the company who sold the code, they said they are aware of the problem and in contact with EA about it, they send me a language pack to copy into the game data.

So I installed the game (in Polish) and copied the language pack into the game data.

Unfortunately, even with a dictionary, I wasn't able to find the language settings and change them.

So I looked into this forum and found out how you change the registry to other languages.

I tried both German and English but when I want to start the game again it takes I while and then says I AM NOT ALLOWED to play Sims4 in the installed language.

Excuse me?!

Does this mean I have to buy a complete new product key that definitely contains the German version?

Thank you for your help.