The other multi-language articles cover building a translated experience. This one covers what happens when a host runs a session.
The host chooses the language
When a host sets up a session, the setup form shows a language picker under Select your language, listing the experience's languages. Whichever they pick becomes the language of that session.
The picker only appears when there's a genuine choice — an experience with English alone doesn't show one.
A session runs in one language
Learners don't choose their own language. Everyone in a session sees it in the language the host picked, so a mixed-language group needs more than one session.
If you're running the same experience for several regions, run a session per language. Each session's report, and each session's post-experience emails, follow that session's language.
Where a translation is missing
Nothing is ever blank. Any field that hasn't been translated shows the English text instead, and any media without a version for that language plays the English file. This happens per field, so a session can be almost entirely in German with two untranslated questions in English.
To see exactly what would fall back, turn on Only untranslated in the Translation tab before the session. See Work in the Translation tab.
Subtitles are chosen separately
Subtitles aren't tied to the session language. Where a video has subtitle tracks, the host can pick which one is showing from the host controls. That choice is separate from the session language. See Subtitles.
Pre-set the language in a link
To pre-select the language on the setup form, add ?language= and the language code to the end of a host link. It's a starting point, not a lock: the host can still change it before starting.