Welcome, Christian!
The ISO language code should be what you want(CH, right?), which is set at the top of your catalog file when running instaUp2Date, or passed with the -i flag when just running instadmg.bash.
A newer product has been developed that might be more straightforward for your needs called SIC, located here:[url]http://mondada.com.au/downloads.html[/url]
It has only been publicly released recently, but helps those who would rather pull the factory image off of new apple gear and make modifications on top of that.
I believe you would have to specifically add the keyboard layouts separately, perhaps on first-boot
Pardon me that with both of the above I have no real-world experience, but did ask around in the ##osx-server IRC room and got confirmation it had worked recently.
Allister
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