Hi,
You can make the "/admin" be a
global route that will bypass the language in the url so you can always access it without the language.
If you really want to have the language on the url even for the admin page like "/lang/admin" then you'll probably need to
adjust your f3-access rules to allow the language slug on the url.
I'm guessing something like:
allow /*/admin= superuser
Hope it helps.
Cheers