Thread:Talk:Universal Language Selector/What is the status of permitting anonymous users to switch interface language?/reply

You don't seem to be using squid or anything, are you? Then ULS just works for everyone, as on translatewiki.net (you can see its configuration files online). With squid, varnish etc. ULS just doesn't work; this has never progressed at the WMF because those who manage the caching system and those who develop ULS never reached an agreement on the appropriate solution, so it was never developed. I don't know if there are news from WMF.

As for temporary language changes outside preferences, that's what the uselang trick is for.