From time to time, we see Garmillans speak but sometimes in their alien language and sometimes in Japanese. Is this just to add flavor (and we should assume it is always the Garmillas language which is spoken)? Or does this dual dub have some setting/plot significance?
I have not seen the anime but a foreigner (or in this case, alien) speaking in the native tongue of the audience or the intended audience is common not just in anime but also in movies. It's a trope called Translation Convention and it is for the 'audience's benefit.' As mentioned in the wiki:
We are meant to assume that the characters are really speaking their own native tongue, and it is being translated purely for our benefit.