If you are using an app on a tablet or phone, it is usually installed in the language of your device.
In the apps: select the wheel icon (top right in the apps), then select the General tab, scroll down, and select the language for the interface.
In GeoGebra v.5, select Options->Language
In GeoGebra v.6 select the menu icon (three stacked segments), then Settings, chose your language in the appearing dialog window, and Save settings
I presume you're working in Classic 6 or in one of the apps: Open the menu (click on the three stripes icon) select the Settings (Einstellungen) and chose the language.
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