Wikimedia Commons has its own custom language selector enabled for anonymous users.
After you choose a language other than English, ?uselang=foo is automatically added to the URL of any links you click. (source code: https://commons.wikimedia.org/wiki/MediaWiki:AnonymousI18N.js). All page views with ?uselang= are uncached. So, for Commons, enabling anon USL (and making pages with the ULS cookie set not cached) would should be fine.
- The currently used (and old) js hack has a outdated style, it won't hurt using ULS.
- Current situation is confusing for users: Having a (functional but old-style) language select in the navbar and an non-functional USL (who tells "Log in to select a different language for menus." when clicking on it).
- The JS language select menu is flipping when loading the page, USL won't do that.
- The whole Wikimedia Commons would look less hacky for anonons.