I have a Survey that is running for 15 days and already has 3000 responses, the base language for it has been English, I now want to add another language to it. Can I do this, and are their any precautions that I should keep in mind.
Yes, you can add a new language to an active survey. In the world of LimeSurvey precaution translates into test everything on an additional installation with the used version.
