Include language in the cache key
The feed for recent changes is cached (for at least $wgFeedCacheTimeout seconds). The cache key considers limit, hideminor (and target), and the code prevents tries to prevent other options from having effect (stating “Feed is cached on limit,hideminor; other params would randomly not work”).
However, the user language, and the uselang query parameter still have effect. That means that the feed gets cached in the preferred language of the user who triggered its rendering, and that language is served to all users during the cache timeout period. (Check e.g. http://ab.wiktionary.org/w/index.php?title=Special:RecentChanges&feed=rss&uselang=ru and immediately afterwards just http://ab.wiktionary.org/w/index.php?title=Special:RecentChanges&feed=rss)
There are, in principle, two ways of solving this bug: 1. Fix the feed so that is always generated in the content language, no matter what the user wants. 2. Change the caching key to include the language.
The first option would not only be user-unfriendly (see also bug #17868), it would also be quite difficult to achieve. Not only does the preferred language affect the feed format, it also affects contents of displayed log events, etc.
Since the second option is also easy to implement, so I am providing a simple patch.