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 switching to another language once the student has entered the exam. If the student needs to switch languages, the exam must be submitted. Then, the student can switch to the correct language according to the instructions above, and then re-enter the exam with an individual exam key.

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