When you start studying a language the so-called “false friends” (words that sound similar in your mother tongue but that have a different meaning) are among the first things you learn. At the beginning they seem difficult but their deceptive nature lends itself to useful memory tricks. Quickly they become your allies and you absorb them.
What they don’t tell you1 is that later, after you’ve become fluent enough in the new language, the false friends come back with a renewed energy and they hit you from behind. This time with a twist: they make you bad at your mother tongue2. I lost count of the times when, speaking in Italian, I said “pretendere” (which means “to demand”) when I wanted to say “fingere” (“to pretend”), or “parenti” (“relatives”) when I wanted to say “genitori” (“parents”).
I have heard other bilingual/multilingual people complaining about these and other longer-term, unexpected failures of the mind. A multilingual friend of mine once told me: “speaking more than one language means speaking all of them mediocrely”. Amen.
[1] Among other things.
[2] They’re not the only reason you start losing your confidence in your mother tongue, but you see them first.
Innumerabilibus 