Let visitors pick the language they answer in
You could already write a survey in every language your site speaks. What you could not do was let the person answering it choose. Whichever language the embed asked for was the one they got, and if that was wrong, their only option was to muddle through.
Switch on "Let visitors change the language" in the survey's Settings and the survey grows a small picker in its header, offering exactly the languages your site is set up for. Pick how it looks: the language names written in themselves - English, Deutsch - or short codes like EN and DE, or just a globe icon. The list always spells the names out in full either way, because a code is enough to recognise the language you are already in and not enough to find one you are not.
Changing it is safe mid-answer. Everything already filled in stays exactly where it was; only the wording around it changes. Nothing is remembered between visits - the next person to open the link starts from your default again, which is what you want on a shared device.
The language each visitor actually answered in is recorded with their response, alongside the survey version that served it. That is not the same thing as the language their browser is set to, and now you can see when the two disagree.