Hi Antonia,
I'd say
While European and American schools favour freedom in reading, which brings significant results, in Croatia there is no systematic research that would determine which books are adequate for which phase of a reader's development.
However, the meaning is not great here. What I understand you are saying, simply put, is
While other countries favour freedom in reading, Croatia does not know which books to tell a reader to read.
The two parts of this sentence don't seem to match, to balance. I hope you see what I mean? So, I'd say something like this.
While European and American schools favour freedom in reading, which brings significant results, Croatian schools do not. In Croatia, there is no systematic research that would determine which books are adequate for which phase of a reader's development.
Clive