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According to the online help you can choose between English and German in "Konfigurationsassistent"
Not sure if this affects manual konfiguration too but I suppose it.
On the other hand it's not hard to understand the german terms if you know what to do.
If you walk through "Konfigurationsassistent" (on the mainpage klick the first Item) and create a new configuration with "Überschreiben" you should be asked for the language.
If you have already configured the PBX you can set the language at "Serverkonfiguration". Klick "Serverkonfiguration" on mainpage. At the bottom of the right page you will find the language dropdown.