For many assignments, especially when the source text is written in the standard literary standard of your source language, it should be safe to use an equivalent neutral and grammatically correct standard in English for your translation. However, for the first time early this year I received the criticism that my translation of a brochure for an airline used language that was too formal/old-fashioned, at least in the opinion of one proofreader. (For example, they didn't like the usage "to elect," in the sentence "The customer can elect to check in at the counter").
But there can be those texts that call for the use of non-standard language, such as chatroom transcripts, e-mails, and other electronic communications. Slang can be notoriously difficult to translate owing to its transience. If you choose to translate slang with slang in your target text, how will you know if your reader will understand it? Perhaps a footnote or an explanation in square brackets is a preferable strategy here?