I imagine Japanese is probably the "last mile" for machine translation given the nuance compared to other languages.
It’s not just the language itself but the culture. Same words can be used differently or have different meaning.
Another thing I notice is that, if a translator is not familiar with how people in a certain culture interact, and what words they use in different situations, they can end up doing things like translating something as a polite, gentle explanation when it's clearly a mocking, condescending one. The literal meaning of the words is there, but they haven't picked up on the tone. Even English does this- "I beg your pardon", "With all due respect" and "As per my last email" are frequently (but not exclusively) hostile even though the literal words are formal/polite. This is also hard for AI to deal with.
https://8020japanese.com/politeness-levels/
It is cultural indeed, but it has impact on the language. Can you cite me another language with so many rules for politeness ?