Yahoo! Posts an Interesting Illustration of the Lang Attribute.
In the post announcing that Yahoo! search results now has natural language support, the YDN blog offers up two audio samples that illustrate a screen reader reading mixed language text with and without the lang attribute. As you can plainly hear, the lang="fr" attribute makes a great difference in the performance of the screen reader when handling mixed language text.
As they point out, the attribute also allows search engines to more easily parse stop words, so there’s an SEO benefit as well.
The lesson here? Polyglots unite! In using the lang attribute.
(Now I run off to add it myself to all the French,Italian, etc. phrases I’ve got littered around the site.)