QWERTZ seems to work better with the (not officially released yet) SFOS 1.1.9.28 (aka SFOS 2.0). At least I can type Umlauts now when the Layout is set to german in the normal Jolla settings.

On the downside it seems that a lot of other keys change to the "standard" layout, as it would be on a normal PC keyboard. For example the "-"-Key right of the number keys now writes a "ß" (which would be at that position on a normal keyboard). Same goes for some other keys, and also the fn-symbols (blue) don't match the characters produced anymore in many cases...