Start by navigating to the survey which you would like to edit by clicking Surveys on the navigation bar and opening your survey in the survey menu.
Once you are in the survey editor, navigate to Language on the bar on the left. This opens up the language pane, in which you can see the languages selected for your survey. You can add more languages from the premade alngauges by clicking on the Add language.
If the language you would like to use is not among the premade ones, you can click Create language in the same drop-down menu where you would choose a language.
This opens a window in which you can customize all the default text in your survey. Near the top of the window, you can set a Name for the language, select the language (ISO) code and select a source language.
The source language is useful for example if you would like to localize the survey for a specific dialect of English. This way you only need to change the words or sentences that are different rom the base language.