But when I try to correct them, they rudely tell me not to do so.
That settles it.
Their argument is that they don't need to speak perfect English, it
just needs to be good enough for the customer to understand them.
It doesn't matter.
You're not their manager. You're the intern. It doesn't matter (even if you're right).
Most likely, you're a student, and you'll probably be moving to a higher paying job once you graduate, and those colleagues will probably be at that company long after you're gone.
Even if you tattle on them to management, in a misguided attempt to feel important, the owner/manager of the company will side with the long-term employees. Most likely, management already knows how your colleagues speak to customers, and they've already decided it wasn't a battle worth fighting.
Let your colleagues be. Knowing when to shut up is very important (even if you happen to be right). Not that I know if you're right or not, I don't know that. I'm just assuming that you're right for the sake of argument. If management has decided not to micromanage them, then it's not your place to micromanage any of them either.
If you continue trying to prove your case, to any of them or to management, those colleagues will just end up mistreating you and bullying you.