Well, so we were searching for the wrong thing. You are referring to the privacy message and not the welcome text.
docs.limesurvey.org/tiki-index.php?page=Template%20editor
{PRIVACY}
Inserts the contents of the privacy.pstpl template
{PRIVACYMESSAGE}
The privacy message is shown if you set your survey to be anonymous. The text can be edited/translated in the language files. This can also be changed manually by editing the privacy.pstpl template you wish to use.
All these texts are to find in the en.po.
However, it is split into various phrases like "A note on privacy", "This survey is anonymous" and "The record kept of your survey responses...".
If you won't find it now, I think I will have to give up...
