What are some good casual Discord servers for talking to people from all over the world?

This is an odd suggestion but you could try learning a constructed language. They're designed to be easy to learn fast and the main benefit for your case is that everyone who learns them wants to speak with people around the world like you do.

Try Esperanto for a full fledged language or toki pona for a super small easy one for fun. Just look them up on youtube or search engine and go down the rabbithole. Lernu is excellent for esperanto and toki pona just do flashcards to memorize all the words (only 120 or so) then go into its discord.

Both have very active discords and people are very willing to talk to practice the language. I know its an out of the box suggestion but its a fun hobby and you can talk to people from tons of countries!

Also of course you could learn a natural language too but thats a much bigger time investment before you can communicate well.

Lemmy.world discord ;D

This one I run.

While also being a casual place with over a few hundred participants and counting, I try to make it serve every function I can think of while not causing it to become authoritarian (it has many rules but that's only because of all the functions, the rules themselves are not that strict and would be befitting of Lemmy cultural attitudes on not being tense) or closed-minded (it allows people of all politics, faith, ethnicities, etc. so as long as they're civil and respect Discord TOS). Anyone here can also promote their own Discord servers (or anything else) in the group if done in relevant categories (the promote category would be where everyone can see it).

If you are up around European evenings then Seemore is one my wife and I use.