European languages
I totally understand what you mean. I use my knowledge of Spanish trying to make sense of French. Actually I often use my knowledge of English to speak Spanish. I almost feel like E and S are identical languages, just regional variants, like Aomori-ben and Kogoshima-ben.
It is sort of amusing to call them different languages sometimes. The other day, the casher girl in the cafeteria was kicking the cahser thing to open it and said "we need a trick to open this thing." I asked her, since she is Mexican, what the word is in Spanish for "trick." Answer was "truco" (or something similar).