I don't think it's nationalism per se, more just a national identity. You see independence movements across Europe (Catalan, Wales, Cornwall, some of these have more realistic prospects than others) that tightly bind the idea of nationhood to a collective language - we are all one people because we all speak the same language. And similarly, when larger countries want to suppress these independence movements, cracking down on their ability to learn or even speak that language is often a key tool used to do that.

Not really, no. AAV isn't just a form of slang, it has distinct and measurable grammatical differences from standard American English, such as its own verb tense, or a requirement for negative concord. You can measure these sorts of features quite clearly, and they are distinct from slang.