NOBODY is impressed by a foreigner who speaks your language with a slang. EVERYBODY is impressed by a foreigner who speaks your language clearly, politely, flawlessly, and putting you to a gentle shame for your own more laid-back approach to your mother tongue.

Nevertheless you will acquire a little slang naturally due to your teacher's speaking. I speak Spanish with a really little Buenos Aires accent. But I try not to, and you really shouldn't care about any slang. Try to be perfect instead, in the process of trying to become perfect you will get the best out of you.