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.