Maybe to _you_, don’t assume other people are the same way.
Along my whole career path (I’m a senior now) I’ve always been curious about the high-level technical stuff and took every opportunity to listen to knowledgeable people.
I think there’s a term for it: incidental knowledge transfer.
You might be stripping that person of the opportunity to grow, or maybe just to hear about something interesting for them to follow up on later.
As for "incidental knowledge transfer" as you call it, there's another side of it: it's called distraction.
Most definitely yes. But she never really shared with us what she felt. So when I heard from my boss she wasn't OK with us talking Russian I was like: WTF, how doesn't she understand she has a privilege to speak her native language at the office while most of the employees have to resort to the silly form of English they learnt at school?
If she felt like the right choice was to bring it up with management, it would seem fairly clear that she felt you were speaking Russian often enough that it would be an imposition to communicate with you, in a way that's quite common in Berlin and in Berlin startups. Though obviously not as much as German, it would be the common crossover language.
If you're senior and they're junior, part of your job is mentorship, and you should be making it as easy as possible to consult with more experienced people if necessary.
Those are the general principles I've always followed, and would do so regardless of whether they seemed incompetent or not. You need to be able to set your ego aside.