Ok, I found a way that worked for me!

If you use sailfish-qml to start your PyOtherSide/QML application you have to place your qm-translation files in a folder called translations.

Translation files are supposed to be named "<appname>-<lang>.qm" in "/usr/share/<appname>/translations/"

That works for me, found it at the source code of sailfishlib project:
https://github.com/sailfish-sdk/libs...shapp_priv.cpp