"I'd like to know how to say "in + a language name""
Whether it's -ul or -ül depends on the last vowel of the stem, it seems.
I don't know about such a rule (it may be a good 'working' one). Where does it come from?
In general, the choice is made according to the vocal harmony of the stem.
Otherwise, how would you decide in the case of 'kínai' ending in 'i' which is considered either a magas (front) vowel or (rather) one that is not 'decisive' (neither front or back, so does not decide about the vocal harmony of a word)?