Are we sure he was speaking dula? The words sound similar yes, but brandon could also just be re using bits of languages that are still whirring around in his head.
Also, did we establish somewhere that Ruin and Preservation created humans from what they had seen before (or, I guess, what the used to be). Perhaps the similar languages ended up on separate worlds in the same way, they came originally from the shards. It's a loose guess, but just an idea.
Also, as per them losing there "Elantrian-ness" (can we find a better word), my whole point is I don't think that is possible. When they go to Teod, their powers are SIGNIFICANTLY weaker, and yet their appearance is, as far as we know, EXACTLY the same. If it were possible to change back in appearance, you think it would have happened at least slightly on their journey to teod, perhaps a slight darkening of the skin, or hair, but we saw none of that. Aon dor is involved in the CHANGING of a mortal into an Elantrian. Once they are an Elantrian, they are one. Getting cut off from aon door completely turns them into leper style elantrians (as we saw with the "It was so beautiful" man in the book). The interlopers were not Leper style, so unless they can work illusion on Roshar, I don't think the seekers could be Elantrians.
I know the man isn't clubs by the way, I was just trying to make the point that similarity does not mean two people are the same