To make your survey bilingual (or to add more than one language), follow these steps (you can even do this while your survey is active).
Let's assume that you've started your survey with English as the base language, and you want to build out your survey in French as well.
1. Go to your survey
2. Click the menu option Survey Properties
3. Click General settings
4. On the right, under the General options
panel. 2nd field down is Additional languages
5. Type French into that box, and you'll be able to select that language.
6. At the top right of the page, click Save
Now, when you edit your questions, and other texts, you'll see two tabs: English (base language) and French.
Click French to enter your French text. Note you'll want to add French text for your survey summary details, your questions, and your answers.
Here's where you set your base and secondary languages:
Once you add additional languages, tabs will appear anywhere that you should enter translated text: