Any language that no one uses. As soon as spoken language is actually used by people who aren't laser focused on ensuring that they don't misuse words or relax

Any language that no one uses. As soon as spoken language is actually used by people who aren't laser focused on ensuring that they don't misuse words or relax pronunciations, the meanings of words will shift and accents will form. People will also invent slang, as people are naturally inclined to do, and some of that slang will stick around in common speech and be accepted as regular words.