[Bug]: Undefined array key 0 at ***/apps/settings/lib/Settings/Personal/PersonalInfo.php with reduce_to_languages option

⚠️ This issue respects the following points: ⚠️

• This is a bug, not a question or a configuration/webserver/proxy issue.
• This issue is not already reported on Github OR Nextcloud Community Forum (I've searched it).
• Nextcloud Server is up to date. See Maintenance and Release Schedule for supported versions.
• I agree to follow Nextcloud's Code of Conduct.

Bug description

After configuring the reduce_to_languages config.php option and having a user which had already configured a now not anymore allowed / available language the given error below is shown in the logs.

Furthermore the following is shown on the personal setting page of that user:

instead of the expected:

Steps to reproduce

1. Go to https://***/settings/user
2. Choose e.g. English (Britain English) or any other language not set / configured in the next step
3. Set / add 'reduce_to_languages' => ['en'], to config.php
4. Go to https://***/settings/user again
5. Check log files

Expected behavior

1. The error in the logs is not showing up
2. Optional: The user is get a note / warning on the personal setting that e.g. a different language should be used and the current language is not "allowed" (not sure if the setting should disallow the language in general)

Nextcloud Server version

30.0.6

Operating system

Should be irrelevant bug can be provided if really required.

PHP engine version

Should be irrelevant bug can be provided if really required.

Web server

Should be irrelevant bug can be provided if really required.

Database engine version

Should be irrelevant bug can be provided if really required.

Is this bug present after an update or on a fresh install?

Update

Are you using the Nextcloud Server Encryption module?

Encryption is Disabled

What user-backends are you using?

• Default user-backend (database)
• LDAP/ Active Directory
• SSO - SAML
• Other

Configuration report

Should be irrelevant bug can be provided if really required.

List of activated Apps

Should be irrelevant bug can be provided if really required.

Nextcloud Signing status

Should be irrelevant bug can be provided if really required.

Nextcloud Logs

{"reqId":"***","level":3,"time":"***","remoteAddr":"***","user":"***","app":"PHP","method":"GET","url":"/settings/user","message":"Undefined array key 0 at ***/apps/settings/lib/Settings/Personal/PersonalInfo.php#280","userAgent":"***","version":"30.0.6.2","data":{"app":"PHP"}}

Additional info

No response

Start in apps/settings/lib/Settings/Personal/PersonalInfo.php around line 280 and reproduce the issue by selecting a language, setting reduce_to_languages to ['en'], and revisiting /settings/user. Trace the handling of a previously selected language that is no longer allowed, then verify that the log error is gone and the personal settings page handles that state as intended.