You can change the language of the general texts of the briefings, per briefing. To do so, follow these steps.
Within your event, go to 'Briefings' in the left side menu.
Click on 'Briefings'.
Search for the briefing you want to change the language for.
Click on the reference name of this briefing.
Click on the button on the top right side with the three dots.
Click 'Edit briefing'.
Open 'Options'.
Tick the box for 'Advanced settings' .
At 'Language', choose the language you want for this briefing.
Click 'Save changes'.
That's it!