A survey in several languages shows a language switcher: a list on the welcome screen and a language menu in the top menu of the survey pages. If participants should stay in the language of their link, you can hide both with one line of CSS in your own theme. Participants then get the language of the link you send them. See Run a survey in several languages with Quick translation.
Add this line:
.form-change-lang { display: none !important; }
Click Copy to local theme and save changes.
Both switchers carry the class form-change-lang in Fruity TwentyThree, so the line hides the list on the welcome screen and the menu at the top.
The language of the link they open. Send each group the link of their language, which you find on the Share page of the survey.
Yes. The change applies to every survey that uses your theme. Create a separate theme for the survey that should hide it.