To add/remove languages to/from your event, go to the Event dashboard module (at Event level) -> Settings tab. There you will find a "Languages" section, in which you can search for and add languages, and delete them.
Don't forget to click the "Save" button to save the modifications you have made.
Once those languages are added, they will become available in both the Email marketing and Website Builder modules. There you can quickly switch between the different localizations available.
If contacts/participants selected a Preferred language in a preferred language field, the website and/or emails and forms will automatically be shown in the selected language. If not, everything will be displayed in the event's default language.
The default language of the event is the first one on the list in the settings tab of the Event dashboard.