You can not change the report language. If you use German version OS then the PBX will generate reports base OS language. The only way to change the language report header is to uninstall the PBX and install it to English version OS.
Deployment Image Servicing and Management tool
Version: 6.3.9600.17031
Image Version: 6.3.9600.17031
Reporting online international settings.
Default system UI language : en-US
System locale : de-DE
Default time zone : W. Europe Standard Time
Active keyboard(s) : 0407:00000407
Keyboard layered driver : PC/AT Enhanced Keyboard (101/102-Key)
Installed language(s): en-US
Type : Fully localized language.
To be honest, I don´t know. It was already setup with 12.5 when I took care of the server. Probably DE. With 12.5 it was no error, because you could choose the language of the report. I´ll report if a reboot helps, when I do one.