a language that sounds like another doesn't mean that they're similar or use the same words. cassian should not be able to speak with sombra or any of the other spanish speaking characters in spanish. they come from earth. they speak spanish. cassian does not. however you want to try to excuse it, the languages aren't the same and him speaking it isn't canon. so either she's doing it for representation points, to shove her way into cr with spanish speaking characters, or because she has no ic/ooc divide and is trying to practice the spanish that she's taking in real life with a character who doesn't speak it.