I think it would depend on the meaning of the word. Does the slang have a hateful or derogatory meaning, or is it a slur? If so, then no. If it has no hateful or derogatory connotations, then there's nothing wrong with it.
Technnically speaking the word 'OK' is slang and nobody gets upset about that.
Yes, in an informal letter it is OK to use slang.
No. It is slang and should not be used in formal English
OK is a slang word, and cowboys used it.
Slang-similar to ok
It means "ok" It is a slang-y way to say "all right."
All right IS American slang - you might also say "OK."
you so fly
It's more a case of no one saying that they weren'tallowed.
You use slang in casual conversation, especially with friends.
Everybody uses slang.
Everyone uses slang. Name any famous person -- yes, they use slang, too.
Ok, first Catholic IS Christian He is a Christian but he never stated what type.