Translation of custom pages (.ui files) does not work

I set up a Qt Installer project and added a custom page and a installer qs script. this part works fine. now I added a translation for the UI file, for example, I have a button with the text "Click me" and I want to add a german translation to "Klicke mich".

first, i created a TS file:
lupdate mypage.ui -ts de_de.ts
I wrote the translations with the linguist and set the fileproperties to "German/Germany". next, I created the qm file:
lrelease de_de.ts -qm de_de.qm
the qm file is located in the packages/mypackage/meta directory, in the package.xml file, I added

<Translations>
<Translation>de_de.qm</Translation>
</Translations>

the binarycreator with -v options shows that the file is copied successful into the installer. If I run the resulting install file, the UI is not translated. Other translations from the xml file are translated correctly, for example:

<Description>My Application</Description>
<Description xml:lang="de_de">Mein Programm</Description>

If I use the UI file in a standard Qt Project and load the qm translation, it works, so the ui and the translation should be correct.