A nice idea: to pull ULS common languages from the Babel box.
This is already done in Wikidata for typing labels and descriptions, and should be done in ULS, too.
See also T26193: Highlight interwiki for the language of homewiki of a user which has a global account, https://www.mediawiki.org/wiki/Universal_Language_Selector/Design/Interlanguage_links/Talk_archive_March_2014%E2%80%93April_2016#Default_language_selection