Hi !
export LANG=pl_PL.UTF-8
./UE4Editor
Create a new project, e.g. “Blueprint/Third Person”.
When UE4Editor is opened, everything works correctly. Then open a file dialog window (e.g. import an asset).
If you work with editor now, you can see that some windows may be empty (e.g. animation editor, blueprint editor, etc).
Close the editor, save all changes. Reopen the editor (with Polish language of course). Open the previous project.
Notice, that you cant rotate the viewport and the “Content Browser” is empty. Click “Window->Content Browser->Content Browser 2” - this works OK.
In “Content Browser 2” right click and import an asset. You can close the file dialog without importing anything. Then try to open e.g. animation editor - The window is empty.
Now if I change something in “Project Settings”, the DefaultGame.ini or DefaultEngine.ini will be destroyed and the game e.g. can’t Cook.
The problem is that in Polish language we use comma “,” for decimals, so “Unreal Engine” stores decimals with “,” instead of “.” in *.ini files (it happens after opening a file dialog - maybe it changes something in float-to-string conversion).
When file dialog was open, go to e.g. “Project Settings->Rendering” and change some values. Then open “DefaultEngine.ini” from game project directory in text editor and you can see commas instead of dots in decimals.
Thats all! Changing LANG to en_US.UTF-8 helps, but it should be fixed anyway 
If it doesn’t work on your system, I can try to record a movie or try to fix it by myself.