The other factor to consider: no two autistic people are alike - one doesn't necessarily have the SAME native language as another - they're just both different from neurotypical. (I have a daughter on the spectrum)

Imagine visiting a new planet where every household has it's OWN unique language, most of them at least somewhat different from all the others, but they can mostly all speak passable english - is it easier for you to learn each of their languages, or for them to "mask" and speak to you in english?