All of the instructions on the website are currently only in English; our mobile app on iOS and Android is localized for Spanish and mandarin language sets. For English language studies seeking international respondents, English does not need to be your first language. Our clients understand that your English may not be perfect if you are not a native speaker.
A good rule of thumb is this: if the study questions are in English, please answer in English. If the study questions are in a different language (Spanish, Japanese, Portuguese, etc.), then we'd expect that you answer in that language. As always, if you have questions, send an email to firstname.lastname@example.org.