This will be a fairly large task.
|Open||None||T49145 Formally deprecate jQuery UI after we've stopped using jQuery UI in extensions and core (replacing it with OOUI).|
|Open||None||T100270 Replace use of jQuery UI and MW UI with OOUI across all Wikimedia-deployed extensions and core|
|Open||None||T122014 Convert all extensions/skins to OOUI|
|Open||None||T158275 UniversalLanguageSelector language filter jQuery UI dialog should converted to OOUI dialog|
The language selector dialog is mostly implemented in the upstream jquery.uls library. I am doubtful whether other users of that library would continue using it if it pulled the OOUI dependency.
Having said that, there is a lot of technical debt in this code, and it would be good to rewrite it. Who knows, maybe it is possible to write the code in a way that having alternative UI on top of the functionality is easy and low maintenance.
Also, there is an about year delay before any new OOUI features and bugfixes can be used ULS and other extensions in MLEB, because master branch of those extensions need to support the two stable releases.