The Universal Language Selector ('ULS') is used as a shortcut to allow users to quickly set language preferences without visiting Special:Preferences.
Currently on production, if a user has set their global preference of interface language and has not enabled a local override the ULS will not successfully set a local override and will therefore fail to display the newly selected language.
Steps to reproduce
- go to https://meta.wikimedia.org/wiki/Special:GlobalPreferences
- set language globally and save
- go to wiki and try to change your language through UniversalLanguageSelector
Result: page get loaded but language is not changed.
- Changing the language preference in the ULS should enable a local override for that wiki, if the interface language preference is set globally.