I'll repost my topic to the english language forum in the hope to catch one of the people knowing the answer:
If I want to use the Leonardo as keyboard controller, how do I have to change the supported language? I changed the core to be able to send straight HID codes and replaced 0x0409 by 0x0407 as entry in the list of supported languages. Still, X11 doesn't accept the german layout given in /etc/default/keyboard (works on Win7 and Debian). What am I missing?