On the Settings tab in your Treejack, OptimalSort, Chalkmark and Questions studies, you can pick which language you’d like your study to be in. 

You can run studies in 70 different languages. The language you select here will be the language your headings and buttons will be displayed in.

You will still need to write your own pre- and post-study questions, tasks, and instructions in the language of your choosing, as the default text is in English.

Use 'Preview' to see exactly which parts of your study are in the language you select.