(I am not a linguist, so please forgive any wrong term or concept)
This question comes from personal experience. I have studied in a trilingual university, so that each student knew at least three languages, usually more due to the high internationality. Most of us were not born bilingual and had to learn the L2 and L3 in school or by ourselves, ending up with different levels in each language. In fact, the L1, L2, and L3 were not the same for everyone, but could vary. As a result, when communicating with each other we had to adapt our vocabulary to the person we were talking to, that is, often simplify it if we had different L1s.
Overtime this ended up in us having poorer vocabularies in our mothertongue. I believe this has happened also because we often had to simplify the logic behind our reasoning so that the other person could fully understand us. We had to use simpler terms and simpler arguments, and finally we adopted these "thought structures" even in our everyday thoughts, having a loss or reduction of their complexity.
I was wondering if there are any studies on the matter. Could it be that a consequence of a multilinguistic society, where people are not all at the same language level, is a loss of complexity in language?