As a student, you can choose the preferred user interface language on the Exam.net start page before entering an exam. It is also possible to change the language in the upper left corner of the student information view:

Please note that it is not possible to switch to another language once the student has entered the exam environment. To change the interface language, the exam must first be submitted by the student, after which the student resumes the exam and switches to the correct language according to the instructions above, before entering the exam again.
Here's more information: Resume the exam as a student

Change language in GeoGebra
For GeoGebra, there is also a setting for language selection in the top menu.