Multi Domain with Admin Tool?

I have a Grav setup with the Admin Plugin.
The site has multiple languages and each is accessed via a separate domain.

So I tried to setup this but failed on the folder structure which was not updated automatically. I also explained the issue here.

Here what I did step by step:

1. setup grav with admin plugin
2. create index.php on root.
3. copied the code of setup_subdomain.php of this Tutorial
4. created the folder 'sites' in 'user/' because I didn't know if I had to (it's now empty...?)?

So I'm really confused on what do I have to do in the 'user/' folder. Do I have to move any folders into 'user/sites'?

Important info: I'm currently working locally with XAMPP.

Multisite setup is a very advance topic, and it's not 100% compatible with the admin plugin. I would not suggest it if your a novice with Grav or if you need to rely on the admin plugin.

You are better off just creating multiple vhosts to one Grav installation with multiple language support. Each entry vhost URL will be treated as a separate 'environment' and each can have it's own configuration, which includes which language is the default etc.

It seems I have a compatibility problem with language support and modular pages.

When I insert the languages in /user/config/system.yaml I get a page not found warning on my modular pages. Error:
PLUGIN_ERROR.ERROR_MESSAGE

I'm not so sure if I should just create the language folders on navigation structure or if I should work with the languages in system.yaml?

I think it would be easier to create a language switcher and all configuration when using the system.yaml.

Please tell me if it's a compatibility issue or if I'm doing something wrong...

Thanks!