On the other forum (StackOverflow) I got the answer from Caleb Huitt - cjhuitt which seems to be the solution/explanatioin to the problem. It goes as follows:

"Static variables are instantiated (and thus, constructor code run) before your int main function is run. The translation code is set up in the QApplication constructor (I believe), which isn't run until your int main function has been entered. Thus, you are trying to get the translation of a string before the code to support it has been initialized.

To avoid this, you could either accept that the given string isn't translated and explicitly translate it every time it is used, or use the Construct on First Use idiom instead of a static member variable."

Thank You for Your time and hope this post also helps someone else.