If I do not understand their meaning, I tell them I don't understand what they were trying to communicate. Using different phrases in different countries/parts of this country or using slang on occasions is not being illiterate. Coming from another country and speaking English enough to be understood is a great thing and as long as I get what they're saying, I would never disparage them; in fact, if their English is good, I compliment them on that, knowing it is a difficult language to learn.
I realize that no one writes casually the way that he/she would if writing an English paper, and I don't either, but a minimal knowledge of the English language is sorely lacking in this country. It shows every day. When we cut everyone too much slack, as our educational system does now, all you get is an illiterate population. I always say, don't be so open-minded that your brain falls out. It's a real trap. If you expect nothing, you get nothing, especially from kids. Knowing how to read and write in your own language fluently is the backbone of a successful life in our country. Everything begins there. Parents and schools both are to blame.
My husband was NCOIC of his unit at Hill AFB in Utah. Professionals would come in and tour from time to time and some of the airmen would use very coarse language. He instructed them that they were to speak in a professional manner on duty, especially when they had "company" coming through, but was told that he could not do that, that the type language they were using was their "culture". He and I strongly disagree with that. Low-brow speak is still low-brow and unacceptable in a professional setting. This is another issue we have here in addition to the dumbing down of the United States as far as our own language goes.