Let me start with small introduction to the problem.
To type a Polish letter you have to make a combination Alt-… where … is aescxzoln which gives you characters like ąęśćźż?łń.
This is called Polish programmers keyboard layout and it’s standard in Poland.
Normally it’s a great solution because we don’t have to use any freaky keyboard layouts like eg. French but just standard US. 
The problem.
Unfortunately in SC when you try to type this way it’s impossible because ALTGR-a instead ą gives you “select all” action so all you typed in the form field before is automatically deleted.
This actually makes SC unusable for creating apps for Polish market.
Fortunately I have found that there is possible to avoid this problem - if you switch off “Highlight Text on Focus” option in form Settings the form returns to normal behaviour.
But it’s not OK if default settings make such a mess.
So, it’s a bug.