Hello Sailors, I finally manged it after reading through your comments it was much easier than I thought.
Indeed you just have to create your ownl qml file:
/usr/share/maliit/plugins/com/jolla/layouts/
I therefore created a
hr.qml
file and simply added additional buttons since I needed more in one row.
Afterwards I added the information for the new qml into the layouts_western.conf:
[hr.qml]
name=Hrvatski
languageCode=HR
and saved it overwriting the original.
After restarting the GUI via Salfish Helper Tools → restart the Startscreen it worked immediately after I added the keyboard from the list of options.
Thank you all very much