Locked instance: Localized name for guest user

[sevonj]

Is your feature request related to a problem? Please describe.
If I enter a locked photon instance, I get the English UI by default (because my OS language, not a photon bug). If I then change the language, the guest user is still named "Guest" in English.

Describe the solution you'd like

• If photon is locked to an instance, don't use the guest user name at all. Instead, use {$t('account.guest')}

Describe alternatives you've considered
-

Additional context