I'm going to demand that a non-native English speaker slow down when they talk to me.
Is there a particular person you want to make that demand of?
My father, he speaks too fast and I never can understand him.
It's fine if you can't understand them and need them to speak slower, but it doesn't sound like it has anything to do with the fact that English isn't their native tongue. You should be supportive of your Father when he attempts to speak in a language that he isn't used to and have more patience, or else he will become annoyed with you.
You're ruining my fun, and I demand that they speak in slow, broken sounding English.
If they really are speaking to fast then it's okay to ask if they can go slower but you shouldn't make demands of people to slow down for just your sake.