MediaWiki developers were happily hacking away while listening to the many interesting presentations. Today they improved the experience for our non English readers.
When people who were not logged in went from Wikipedia to Commons, they would be surprised with a user interface that all of a sudden was in English. Commons is now aware of the language that you used in the Wikipedia.
When you go directly to Commons, the language will depend on such things as the language that is configured in your browser. For this improvement to work well we rely on the quality of the MediaWiki localisation.