The bilingual, when they even bother to indulge the efforts of the English-speaking person in their own language, will probably do so patronizingly and with metered patience.
Sometimes while trying to help, the bilingual will talk so slowly that it's hard to pay attention.
Or, maybe the problem is not the slowness of the speech but the fact that it's not in a natural rhythm. It's as if the speaker is not trying to communicate as much as trying to speak slowly enough that you should be able to understand.
Those who don't speak English at all talk like normal people. One can ask them to slow down, at least, if it's necessary.
But the bilingual? Assholes. And what can you do? If you ask them to speak more quickly, they'll give it to you good. And how do you ask "could you speak more naturally" in the other language? I'm trying to figure out how to express the idea here, even in English.