If you want, for example, to change the text of the next button, you can do this by clicking on the “three dots” on the right hand side of the language you want to edit. This will show all default messages.
When you click on this, a list of default labels / text will apear.
Note: This text can only be edited within Survalyzer and won’t be exported into the translation file. Each change will only be saved to that particular survey. If you copy a survey, these changes also get copied.