Think about it this way: Would it be considered rude if somebody else tried to learn English for speaking with you even though he can not speak so well? I can not see why a harm would come from learning a new language. Even if you cover the basics as the begining.
If , for example, you were German , they were going to do it with German words they knew. Not just that but if they happen to be young, they were going start with kinky words they know and they were laugh about it and ask you if they say it correct.
People , specially young people, do it almost every time they meet with somebody who is not from their culture. Its their way of trying to make a contact and say I know a little bit about your culture. Its silly, but extreemly common. But if it bothers you, I guess you just should ignore them.
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