I wanted to add some way to determine a characters origin (home world like "forgotten realms" and homeland like "Thay") to use these as limitations for some feats and subclasses.
Originally I was thinking to just create a homebrew version of Human with subraces for each nation to allow for these distinctions (as well as more lore canon Languages etc), but it would be more useful as a separate setting since I might want to be able to apply it also to other races than humans. Anyone have any ideas on how to best go about this?

Depending on what you want it to do you might use homebrewed feats. Those can be added directly to a character sheet at character creation to accomplish that. The custom languages will have to be added to the character sheet separately since homebrewed languages aren’t a thing here.

Hmmm, I want to use the homeland as a prerequisite for some feats and subclasses, but seems I cannot use homebrew feats as prerequisites.

You can still list it as the prerequisite, it just won’t be enforced by the system.

yeah, that makes sense. I guess there is no way to make the system enforce it? I don't want to clutter the character creation screen with tons of invalid options.

It won’t, feats can’t be added on the creation screen, they have to be added directly to the character sheet under Features & Traits->Feats->Manage Feats

Oh, that's very disappointing

Do you know if there's a list of tags and syntax somewhere, or do I have to guess and look at others creations?

That depends on what you mean. Do you mean tooltips or snippets?

Both, thanks a lot!! :)