The words 'izoliran' and 'kostur' in the MWE
\documentclass{article}
\usepackage[croatian]{babel}
\begin{document}
mmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm izoliran
mmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm kostur
\end{document}
are not hyphenated correctly: they appear as iz-oliran and kos-tur on my end. Hyphenation for other languages seems to work fine, and mostly agrees with what the Croatian one should be (i-zo-li-ran and ko-stur). For what it's worth, I'm using a recently updated TexLive installation, and what seems to be the latest version of Babel (the documentation is version 3.9l). Is this a real problem with the hyphenation patterns for this particular language, or is something else going on?
As for the solution, almost anything would be better than the English patterns that seem to be used now. Is there a way to use [croatian]{babel} with the hyphenation patterns of another language? Serbian, for example, would be close to perfect.
Edit: what I said about English patterns being used is definitively wrong, so these syllabifications are what they are 'by design'.
\hyphenation{i-zo-li-ran ko-stur}
but that's just for exceptions, this seems to be a larger problem.