@fregatte: On my ancient K3, the device automatically selects dictionaries based on the language code of the book. If you have books in multiple languages installed, it couldn't hurt to make sure that all of them have the correct language code, because in the past it often happened that users complained about dictionary lookup not working, only to find out that the language code of all their foreign language books was set to English.
You can check the language code in Calibre by clicking the Edit Metadata button in the toolbar or by opening the book with
Java Mobi Metadata Editor.
EDIT: To exclude problems with the non-English dictionaries themselves, backup your German-English dictionary on your PC, replace it with Michael Sheldon's GPL licensed
German-English Kindle dictionary and try to look up a very common word with it in in one of your German books. (If you don't have any German books installed, download one from
MR.)