Garmillans only sometimes speak alien language
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.