I often reply under Japanese posts, and I always assume users will use a translator as I do, but maybe in the context of a Japanese this may look rude?

Can’t speak for others (obviously, as this is about individual etiquette perceptions) but I would consider it to be polite to only enter conversations with unknown parties in languages that the parties have shown to be capable of speaking and understanding.
Using a new language entering a conversation would therefore signal either familiarity (“I know they understand me”) or rudeness (“I don’t care if they understand me”) to me, I suppose.