If you're as annoyed as I am that WordPress always claims the language in feeds is 'en' instead of 'de', and you've looked for the corresponding setting in the options: forget it, the WordPress developers have provided the option but not the GUI to set it. Rarely stupid.
Well, you can set it quite easily in a MySQL shell:
update wp_options
set option_value = 'de'
where option_name = 'rss_language'
Or alternatively with PHPMyAdmin or similar tools, simply find the corresponding data record and then patch the option_value field. It would be nicer if WordPress had made it possible to set this via the admin interface ...