How does one steal a translation? You don't own the words or sentences, you find them.
They still put in effort to actually translate it in the first place though. And even if the text itself isn't theirs, their translation might be given how translation is never a 1:1 conversion process from one language to another and requires some consideration from the translator themselves; another translator might've translated the text in the image differently, even if only slightly, and it'd be as correct.
Fundamentally, they should've at least been credited.
I honestly feel the person who got offended needs to drop the ego. A translation is a translation, we appreciate it, we give the translator credit. It's not like we're taking food from your plate or anything.
Sigfried666 said: What if it just happens another person translates a text with the same words. How do we "prove ownership" of a translation?
In general, probably won't be doable.
In this case, bmxeno borrowed translations from equivalent Gelbooru uploads on two Nikke images, both of which were coincidentally translated by the same user. He claimed honestly that they came from the Gelbooru post but not by whom, and KonuriMaki claimed in the comments of both posts here. There's a trail. The only exercise remaining would be KonuriMaki -> Lekso, if you want to press that at all. (I wouldn't.)
Honestly surprised, this is probably a first I've seen someone mad about using their translation on this site. Most translators I've used didn't seem to care as long as proper credit was given.