Maybe it's for preventing unwarranted fucks[1]? Translation is more than just concatenating dictionary definitions, and machine translations routinely make this kind of out-of-place and technically correct lookups.

1: https://www.google.com/search?q=engrish+fucking+sign&tbm=isc...

Meta employee here. The system is not perfect, or it would not "hallucinate", while it's pretty good, it does sometime make errors (not just hallucination, maybe just some mistranslation due to noise in the training data). What we want is to avoid these errors to introduce toxicity (think swear words) that weren't in the input as this could be very bad for the user. There is a separate system that double checks the output (compared to the input) and tells the translation model to try again if it's too bad.