Participants complete tasks in the language that is set in their personal account as the "Olympiad Participation Language" and was selected during registration. Later, you could have changed the language through the Organizing Committee.
Before the start of testing (even during registration, i.e., when selecting a time slot to take the test!), the participant sees the participation language. If the language needed to be changed, the participant should have definitely taken care of this before the testing began.