In America we are trilingual. Although they involve similar sounds, they are quite distinctive in how they convey ideas. Foreigners who have studied Formal English have a lot of trouble understanding local slang. If you're from the neighborhood, then there is no problem. If you want to communicate with someone from another neighborhood, then you use Formal English.
Slang is too volatile and localized for communication with folks from other geographical locations.
Formal English, because it is widely taught, is the best way to communicate with a stranger. If you want to communicte with the court, then you use King's English, also called the Language of the Court, or the Language of the Law. It is a separate language and should be studied as such. Changes in meaning can take as long a 300 years, so it is useful for referencing precedent. Law schools do not really teach the idea that it is a separate language.