I can confirm this behavior with external auth.

For reference, we have the following set in the netbox config:

REMOTE_AUTH_ENABLED = True
REMOTE_AUTH_BACKEND = 'netbox.authentication.RemoteUserBackend'
REMOTE_AUTH_HEADER = 'HTTP_X_REMOTE_USER'

Environment: self-hosted, Netbox 4.0.7, Python 3.12

@salfers I checked it using AzureAD and cannot confirm/reproduce your behavior.

Was your user created by SSO or did it already exist?
Can you please also post your configuration parameters starting with REMOTE_AUTH_ (omit any sensitive data, of course)?

@m2martin In my case the user already existed.
It had it's language selection in trhe user prefereneces set to English, but Netbox showed German upon login, which is (browser language default.
Deleteing the language settings and setting it to English again and then applying the perferences using the save buttons sets English for the current session.

I'm using the following configuration parameter settings:

REMOTE_AUTH_ENABLED = True
REMOTE_AUTH_BACKEND = 'netbox.authentication.RemoteUserBackend'
REMOTE_AUTH_HEADER = 'HTTP_X_REMOTE_USER'
REMOTE_AUTH_AUTO_CREATE_USER = True
REMOTE_AUTH_DEFAULT_GROUPS = ['staff']
REMOTE_AUTH_DEFAULT_PERMISSIONS = {}

With REMOTE_AUTH_BACKEND = 'netbox.authentication.RemoteUserBackend' set, I can partially reproduce it. I can see the first page being loaded with the browser's language but any subsequent request will be displayed in the language from the user's properties.
Can you confirm this behavior?

@m2martin No, I cannot confirm the behavior. The language stays on the browser default and the user preferences have no impact.

What I did
1, open new Incognito-Tab in Chrome
2. open Netbox URL and authenticate using HTTP Auth
3. click on Devices ("Geräte")menu entry in the Devices ("Geräte") section
4. click on the Netbox logo menu

No change in language. I can only get it to switch to English (the users perference) by opening the user preferences and then Clicking "save"

I'm doing the same and can not confirm it. Can you please try it with v4.0.8? This is the most obvious difference between our tests.
(Maybe fixed with #16732)

@m2martin I'm running 4.0.8 since a few days. There was no change in behavior between 4.0.8 and 4.0.7.

We are having the same issue.
Browser language is German, but netbox DEFAULT_LANGUAGE is set to en-us.
Upon login with SSO netbox is in german, not only the first site after login.
In the user settings the language is set to english. Upon saving them without changing anything the correct language is applied.
This resets every few days and the user has to change apply the user settings again.

FWIW, I also have set DEFAULT_LANGUAGE = 'en-us'

I think, I found it. Is LOGIN_PERSISTENCE = False?

@m2martin Yes, I have LOGIN_PERSISTENCE = False

Other settings:

LOGIN_REQUIRED = True
LOGIN_TIMEOUT = None

@arthanson I have a fix ready. You can assign it to me.

Root cause:
Language cookies will only be set on regular logins by the login view or renewed when having LOGIN_PERSISTENCE=True.
Setting the cookie in every workflow for every possible authentication method (local, OAuth, LDAP, etc.) is hard to maintain.
With the fix the language handling was moved to the CoreMiddleware to have it processed independent from the authentication method/backend.

@m2martin I've assigned this to you, thanks.