It’s always good to encourage students to speak the local slang. Understanding how people interact with language is the most important part of communicating efficiently. Not properly interpreting words can lead to confusion, students may not feel like the fit in properly if they can’t understand dialect.
In the overall scheme of things, speaking properly is not as important as communicating in a manner that can be understood. It is hard at times to deviate from teaching what is proper but it seems that lots of understandings now are getting lost in translation. It is hard to fully grasp concepts when the ideas are too abstract, giving concrete understandings of slang terminologies will increase success of using English as a second language.
It’s also important however to make sure you explain why certain words are used differently. People who don’t speak the native language may have a hard time seeing the correlation between certain words and their meanings. That is because the culture you come from may not view certain concepts in the same way. That is why it is always imperative to make sure the student understands in their own language as well.
Like the saying, “That’s gravy.” It seems like you are talking about a sauce you would put on food but in some places you are actually saying, “That is good.” More than likely the translation came from gravy being delicious and when it comes to food – delicious is good.
With simple translations and understandings like this it can be easier to grasp a second language, especially if you know the local area that you want to move to. It’s best not to prepare yourself to speak by rules but to prepare yourself for what you are going to hear and how you are going to be expected to express yourself.