Croat hools in football scene don`t have many friends among other slavic countries. In fact they don`t have what we call friendships, "friendship" or "brotherhood" must be official. Subgroups from our two main tifo groups have connections with other tifo subgroup.
In slavic world :)
BBB have good connections with Levski.
ULTRAS Levski Sofia & BBB Dinamo Zagreb
Torcida have some personal connections (members from polish Torcida went in Split, and members of Torcida Split went in Poland)  with Torcida Gornik Zabrze, but this not the official.
These polish brothers really love Torcida :)
samo tebe volin ja…MOD
This is original from Split.
Maybe there are other groups in Slavic world, but I`m amateur in football and Tifo scene.