Problem with German special character

When i try to use German special character like ÄÜÖ i get the wrong Letters on the GUI.
2020-06-10 01_52_26-Support Dokumentation

on the Powershell Script i have the Correct Letters included, but after executing the Script or the EXE i get the Letters, that tells me the Default Encoding isn’t maybe UTF or something like that.
Also i have tried the following line of code to work-a-round that:
[Threading.Thread]::CurrentThread.CurrentUICulture = 'de-DE'

Maybe someone can help me.