Anecdote from my personal experience in daily life, as a German in Germany.
While I can speak English mostly without any special effort, it sometimes surprises me that I need time (up to 2 seconds maybe) to switch, when someone is asking me something. As in at first it doesn't even register as English, more like thinking what was that?. Then, delayed I suddenly understand. Of course it depends on the situation (environment, other sounds, other speakers, and so on, too) and the accent/dialect of the speaker. In other cases no problem at all, like instant switching without any hassle/thinking about it.
I sometimes experience something similar with English (my first language, and I'm monolingual, unless we're being extremely charitable). I'll ask someone to repeat themselves, then momentarily afterwards, what they said suddenly clicks, before they've gone ahead and repeated the message.
I think of it like this: we have an interpretation subsystem to interpret messages carried in sound (in whichever language). It's a soft real-time system. Typically it succeeds in interpreting good inputs, making its deadline, and it decisively rejects invalid inputs (on account of mumbling, overwhelming background noise, etc), also within the deadline. Occasionally it succeeds in interpreting a sound signal but misses its deadline.
There's also a request-repeat subsystem, which may fire any time after the interpretation subsystem has either rejected an input as invalid, or missed a deadline. When it fires, we say something like What was that? to request retransmission of the previous message.
When the interpretation subsystem misses a deadline but then succeeds, it races the request-repeat subsystem, so there's a chance of a What was that? being uttered immediately before successful interpretation of the previous message.
I have the same, but in reverse. I can get the gist of German speech, if I'm expecting it and have some clue about context. If it's out of context, then it just comes across as a mess of sounds. I find it so strange that a sentence made entirely out of words that I know is so incomprehensible out of context.
While I can speak English mostly without any special effort, it sometimes surprises me that I need time (up to 2 seconds maybe) to switch, when someone is asking me something. As in at first it doesn't even register as English, more like thinking what was that?. Then, delayed I suddenly understand. Of course it depends on the situation (environment, other sounds, other speakers, and so on, too) and the accent/dialect of the speaker. In other cases no problem at all, like instant switching without any hassle/thinking about it.
It depends/varies in strange ways :)