Good.
Open the old Swedish Project, and open Data Set Manager. I think you will find that the one Data Set in that project is called "My Data Set." That's the default name for the Data Set when you create a new project from scratch. I'd suspect you never had reason to notice it and so didn't change it.
If that's correct, then the Data Set called "My Data Set" in the combined Project is your Swedish data. So you want that one to "remain unchanged" when you do the Data Set merge. That means that the contents of that Data Set are copied to the other one. Once you have done that, and verified that the people from the Swedish Project appear twice - once in Data Set #1, and again in DS #2, you can delete DS #2 from that Project.
Thanks, it worked like a charm! Cellobug