Since there is no question here, I will just add some observations:
First point is that this is not limited to the Czech Language Pack.
The first case works the same in the English version. But this may be correct since the you are inserting a line in a paragraph you have formatted with a line that inserted line inherits the formatting
The second case "nothing" - you are adding a bottom border not a bottom line and only for the selected characters and none are selected. However, even if some characters are selected the behaviour is incorrect. In the steps
bbbb"
"<ALT>HBB"
nothing
"<ALT>HBB"
missing previews horizontal line under aaaa
you need to move down to aaaa to see the effects, but they do occur, but if you are on aaaa an the horziontal line is there then removing the bottom border does take these off.
If this answer solves your problem, please check Mark as Answered. If this answer helps, please click the Vote as Helpful button. Cheers, Shane Devenshire