It's equally mind boggling that tweens and teens can understand and speak a made up language but cannot understand or speak a language like Spanish or French taught in school, amirite?

Are you implying that other languages weren't made up? 😏

The goal is not to be understood by adults / in academic settings.

Replacing the words of their mother tongue is quite different from studying the grammatical structure, vocabulary and pronunciations of a foreign language, nevertheless it is still remarkable how fast they can think up and learn of such constructs in comparison to adults

I'm sorry what??

Are you talking about slang that is still based on and structured around English?