Exclude lang="xy" from rss feed

I have my site running with languageCode = “de” as default language, and have postings in German as in English on my blog. I’m not translating anything, so I’ve just used lang = “de” resp. lang = “en” in the header of my posts to pass that into the XHTML headers, that works all right.
However for some reason I want to exclude the German posts from the RSS feed, how could I do that. I’ve tried to use if eq .LANG “en” in the template for the feed, but there’s a failure which says that .LANG can’t be evaluated.
Thanks in advance for hints,

Try .Lang