Jetpack uses your site's WPLANG constant to define the language to be used in the Jetpack menus in your dashboard, and on your site.
Could you try to set your site's language to de_DE, as explained here:
http://codex.wordpress.org/Installing_WordPress_in_Your_Language
You can also download language files to translate the rest of the WordPress interface here:
http://codex.wordpress.org/WordPress_in_Your_Language#German_-_Deutsch_.28de_DE.29
If that doesn't help, could you post your site URL here, so I can have a look?
If you want it to remain private, you can also contact us via this contact form:
http://jetpack.me/contact-support/