Because you want someone to be clearly speaking in a foreign language, perhaps in front of someone who doesn't immediately understand it, or for other plot reasons.
Because in that particular instance (a) showing is better than telling and (b) you want the reader to understand/make a guess at what has been said eg so that they're ahead of the POV character.
I don't know if there's a failure of communication here, but I must say I'm astounded that occasional short sentences in a foreign language are seen as problematic if phrases in those languages -- which might be as long or longer -- are apparently acceptable.
I think my point here is best broken into three points.
1) I have no inherent acceptance of the conceit of "they translated this to English, and translated the foreign languages in it to other foreign languages as well" and see no logical reason to bring it up. It can all be done in English - there are many accepted conceits for doing this. That's the best for easy understanding and acceptability.
If someone wishes to do it differently, then sure, but it's not something I'm going to accept just because. I wouldn't blink at an author using words with clear place/person etymologies in their translation (and some people don't have that conceit and do), but I will if you tell me the secondary world dwarves speak Gaelic or Hebrew because I don't have that conceit.
2) The acceptability in a vacuum
This is hugely context dependent but by and large, yes. I am willing to accept the best translation might involve the use of phrases in their thoughts that get across ideas English doesn't do a well, but the moment they are actually speaking and thinking in another language - not using a phase, but speaking and thinking - then I am going to potentially start viewing the book differently. That's where the emotional switch is for me. And obviously I am just me, but I don't feel like I'm unique here.
3) The acceptability of an author doing it in general
Look, if you or any author genuinely enjoy doing this and think it best for your works, rock on. Got to do what works for you. It won't be what the book lives and dies on for 99.9% of readers in any case. Even people who find it potentially immersion ruining are going to be fine a lot of the time if they're enjoying it.
But for an author who doesn't have a strong preference on how to represent foreign languages in their work - like the OP seems to be - then simply avoiding putting any other languages on the page is the simplest thing to do and also avoids some potentially major pitfalls.