Feature summary (what you would like to be able to do and where):

• When you visit Special:Preferences#mw-prefsection-editing on any wiki with the Translate extension enabled, there is a section for assistant languages. In it, you can enter a comma-separated list of language codes. It would be nice if this used the same type of language selector that you use in Special:PageTranslation, which also used to be a comma-separated list of language codes, but now has a new(er) system.

Benefits (why should this be implemented?):

• Easier input for end users.