You're not alone. I took a closer look. It happens when the slider is moved with the mouse wheel. It's also not limited to the first slider. The corresponding input box is selected. If you have previously selected a different input box then it will affect this slider.
I still can't replcaite the error. Can you give a detailed description of the steps to reproduce the error?
@musko Nothing odd there. Maybe the UI behaves differently with Slovak language selected. If we can nail down some more details about it, and possibly get some more "foreign" language testers, then we can report it as a UI bug.