Ok, I’ve managed to install the current version and in the first place the umlauts were not gone. Then I’ve deleted
echo-database from the pimatic root folder after stopping pimatic. Then deleted the device list from alexa.amazon.com and searched for devices again. But I unplugged my echo dot 2nd gen before. This time the umlauts were fixed. So it might be because of the new
echo-database or because of the old echo dot.
Just in case you have an old echo dot and a newer version, make sure that just the newer versions search for devices.