Hey Luhe, as a dabbler in linguistics, I've thought about the issues you brought up. Yeah, politicizing languages is a thing, unfortunately.
I used to think Chinese was a language, until I found out about Mandarin and Cantonese and the many different mutually unintelligible languages within China all considered Chinese! The only unifying thing they have is the written language.
As a Spanish speaker, I can speak to a Portuguese speaker in Spanish, and they back in Portuguese and we understand each other. That really depends though. Slang is an entire subject on its own. I don't understand their slang at all. Every Spanish speaking country has their own slang as well. I had to learn how to speak Cuban slang in order to really understand my Cuban friends.
As an example, English slang:
What's hanging? What's up, dude?
Cuban slang:
¿Que bolá acere?
Literally means "What are you up to, group of smelly monkeys?" It only refers to a single person though.