Montenegro contains a false friend.* That someone takes offense at it is easily rectified by explanation. See also Niger.
As for people speaking other languages, yes, people get upset about that. Some whenever it occurs, some only situationally.
But here’s the thing: you don‘t really have the power to tell someone not to be offended by something by your mere say-so. Trying to do so has a high probability of simply reinforcing their negative feelings. I mean, have you ever told your significant other to simply “not be angry” about something they were already very angry about? How did that work out?
You can’t control someone’s level of negativity, you can only control your response to it.
* See previous post that was probably edited while you were already responding, or this link:
en.wikipedia.org