An experience carries its own list of languages. That list controls which columns appear in the Translation tab and which languages a host can choose from when they start a session.
When you create an experience
Start creating an experience as usual.
Under the project picker, open the Languages section.
Click Add language and pick the languages you want.
The section is collapsed by default and shows a summary of what's selected, so you can see the languages without opening it. If your organisation also has subtitles, this section is called Language and subtitles and holds both pickers — they're separate lists and don't have to match.
Changing them later
Open the experience and click Settings.
Find the Languages card.
Use Add language to add one, or the × on a pill to remove one.
English is pinned as the base language and can't be removed.
Removing a language doesn't delete anything. It only takes the language off this experience — the translations stay stored, and if you add the language back they reappear as they were.
Setting defaults for your organisation
If you're an administrator, you can set the languages every new experience starts with:
Go to your organisation settings and open Content settings.
Under Default languages, add the languages you want.
New experiences will start with those languages in addition to English. Whoever creates an experience can still change them in the create dialog.
You can set up to 30 default languages. That ceiling exists because it's the most languages your safe words can be protected in.
What happens next
Once an experience has more than one language, a Translation tab appears at the top of it, and a globe appears in the toolbar so you can switch which language you're editing in. See Working in the Translation tab.
Other settings for the experience — title, cover image, participant anonymity and the rest — are covered in Managing experience settings.