If you are running on vRA 6.1, go to Preferences on the right top corner of vRA page. Under the Notifications you can change the language settings.
When I'm on the page /vcac and i'm firstname.lastname@example.org, I open Preferenses, but the error appears: Changing of preferences unavailable..
When i'm on the page /vcac/org/test and i'm domain-user assigned earlier I can open Preferences, but the language is still russian despite I just changed it..and the Language tab is empty in Notifications...
Yes, you won't able to get the Preferences when you are in the default tenant vsphere.local.. and that is fine.
Just to check, what's the browser's preferred language? I guess you might have checked it but still
I have not come across this issue. Even though the user has very minimal rights, he will be able to change the language preferences.