1. Have your system use a non-English UI.
   I use German.
2. Run Creator configured to use the "System Language" for its own UI.
3. Open Creator's sources as a project in the running Creator.
4. To be sure, add parameter "-tcs" to the project's run configuration, so the following is not being affected by existing settings.
5. Run the project.
   Creator from the sources you opened gets started with its UI using the "System Language".
6. Debug the project.
   Creator from the sources you opened gets started with English UI.

I expected this to consistently show the "System Language".