From your Survey summary page, go to Responses->Responses and statistics. Then click on Responses and choose either language (the language you choose just determines what language the column headings are shown in).
Then look for the column startlanguage -- you'll see either "en" or "fr" (if your survey was available in English and French).
The way this value is determined is based on what link they used to start the survey (did they click on the English or French link to start the survey).
If you only shared the English link, and participants then selected French, you'll only see "en" in this column -- this is because they arrived at the survey in English.
Because a participant can toggle back and forth between languages during the survey, there is no way to determine whether they took the survey in a specific language because they could in fact toggle back and forth between languages on each page of your survey.