If you speak English, I'd be happy to assist.

In order for K2 content (and not just K2 - but every Joomla component or module) to work in specific languages (and load the associated language file as well), you MUST assign content a specific language. In other words, German items must be assigned to language "de" and English items to "en" in the K2 backend. Of course you'll have to enable language switching modules in Joomla as well. Is that the case?

Re-install Joomla using the "Update" component and then re-install the language packs, just in case.