To change the language of your interface, go to My profile, then in the Language field, click on the arrow to activate the drop down menu. You can select the language wanted among the available ones: English, French, French (Canadian) Spanish, Catalan, Italian, Portuguese, Estonian, Arabic, German, Chinese, Indonesian, Dutch, Japanese, Vietnamese, and Korean.
The organizer defined the language by default when creating the event, as well as potential additional translations, which means that the event content will be available in these languages only. However, the app content is available in all these languages (control buttons, categories, notifications, etc..).