[Bug]: Cannot change language `conflict between new type (mixed) and old type (string)`

[skjnldsv]

Bug description

On current master, changing language leads to conflict between new type (mixed) and old type (string)
Seems related to #47658

Request payload

Xdebug snapshot:

Stack trace

Steps to reproduce

1. Have language set
2. Try to change it

Expected behavior

No error

Nextcloud Server version

master

Operating system

Arch btw

PHP engine version

PHP 8.4

Web server

Apache (supported)

Database engine version

MariaDB

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

Upgraded to a MAJOR version (ex. 31 to 32)

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