What happens if Google has assumed your page is a different language than what it's written in? Well, I don't think mod_rewrite will work. But a few things will:
This declaration should be placed on the top of all your pages in the HEAD tag.
Furthermore, you should utilize Google Webmaster Tools to ensure that Google is acknowledging the proper language of your site.
What else would you suggest for this user whose site was inadvertently assumed as German even though it's written in English?
Forum discussion continues at WebmasterWorld.