It would be very helpful, both for accessibility and for directories, if WordPress could store the primary language of the blog and include it in the headers for both the XHTML and the RSS, Atom, etc. templates. It could be a set-once-and-forget option, possibly part of the setup routine, with the option of leaving it undefined for multi-language blogs.
This is a Web Accessibility Initiative issue. It can improve standard search results. It can improve feed search results. In fact, it's arguably more useful than lattitude and longitude, which has a nifty "gee whiz" aspect, but aside from locating sites by proximity, it still has to be mapped to city/country/etc. to be particularly informative.
I realize this has been discussed earlier, but thought that perhaps the way it was originally brought up might have distracted from the suggestion itself.