Mrs. Kralilova,
In the past I found the name ISTOK in a Hungarian dictionary and the name meant the cutting edge of the sword, or an 8 inch long knife. I have always taken that in a figurative way, thinking that we are leaders, able to move forward and change thing s for the better.
More recently, my wife and I have been teaching English to many immigrants (legal)in our town, one of which was Russian. He says an ISTOK is a small stream or creek that feeds into a larger stream or river. There is a WISLOW river north of Fricovce that flows northward into Poland. There might be a connection to the names here.
Mark