Spanish

Except Michael Fassbender isn't a native Spanish speaker and it shows. Perhaps they should have dubbed him.

Actually no I don't think it was a great decision. It feels like a pointless change from the games and I think opens up some problems that wouldn't be present had they just kept the translation software in place.

The biggest problem that I see is that either Callum has no idea what is going on which is bad because the Templars need to him to find the Apple in Aguilar's memories (he can desync if he does something wrong) or Callum magically learns Spanish from the Animus via the Bleeding Effect which feels weird after the changed the Bleeding Effect from Animus magic to more repeated muscle memory with the new Animus (which I liked).

That's on top of making it harder to film because you're dealing with non native Spanish speakers so it would have been easier to just have them speak in English with a handwaved translation software and the possibility of taking the audience out of the story by making them focus on subtitles instead of what is going on, on screen.