If you're looking for synonyms, here are some options depending on the context:
Formal:
* Informal
* Unofficial
* Conversational
* Vulgar (when referring to slang or dialect)
* Vernacular
* Idiomatic
Informal:
* Everyday language
* Slang
* Street talk
* Local dialect
* Jargon
* Chatty
The best word depends on what you want to emphasize. For example, "informal" is a good general term, while "slang" focuses on specific, informal words.