Learn English – How to express that you can understand the English that someone spoke

In common conversation in the US Midwest I rarely hear "Not at all" or "Don't mention it." "No problem" is very common, and "You're welcome" is also pretty well-used.

My personal usage:

I use "Not at all," "Don't mention it," and "No problem" when the activity I'm being thanked for was really no big deal. I use "My pleasure" when emphasizing that I'm happy to be of assistance (often in a customer service context), and "You're welcome" when the action prompting the thanks was a little bit of a bother. In essence I use different phrases to indicate how "thanks-worthy" the activity was.

That's probably not common usage, though.

EDIT

I think I misrepresented what I originally meant, so here's a little clarification.

If someone thanks me for something I always do (for instance I always cook dinner in our house) then I would say "No problem" or "My pleasure" depending on context. If I did a chore that was someone else's responsibility, I would say "You're welcome" even if I was happy to have done it, because it took an extra effort on my part, not because it was a "bother."

You could consider using passive speaker as defined in Wikipedia:

A passive speaker (also referred to as a receptive bilingual or passive bilingual) is someone who has had enough exposure to a language in childhood to have a native-like comprehension of it, but has little or no active command of it.

As it is not a broadly used term, you might have to add something after saying it for example:

He is a passive speaker in French. He does understand it very well, but can't speak it fluently.