As someone who has moved to Japan for the last ~12 years and speaks the language daily, I struggle to remember terms I am absolutely postive I know but can't recall while in the middle of conversations with my English speaking employees. Or even worse, deep in personal thought I have to start googling around to recall a term I know I know.
There are linguistic studies which show that this is the norm (i.e. as a second language learner using said language daily, you begin to forget your mother tongue), but I have to admit it is quite disconcerting when I have to take a multi-second break to recall an uncommon word before I can carry the conversation further.
I guess the moral of the story is that, yeah, you forget things you don't actively use.
There are linguistic studies which show that this is the norm (i.e. as a second language learner using said language daily, you begin to forget your mother tongue), but I have to admit it is quite disconcerting when I have to take a multi-second break to recall an uncommon word before I can carry the conversation further.
I guess the moral of the story is that, yeah, you forget things you don't actively use.