Major Description
Right Alt+A (Alt Gr) is a key which simulates Ctrl+Left Alt (see history section in http://en.wikipedia.org/wiki/AltGr_key). This causes all text to be selected when the Polish special character "ą" is entered.
TerraTextInputSkin defines "select all" as <command key>+A:
...
} else if (keyCode == Keyboard.KeyCode.A
&& Keyboard.isPressed(commandModifier)) {
...
On Windows, the command key is Ctrl. The isPressed() method uses a bitmask, so it returns true for the Ctrl+Left Alt case which Alt Gr generates. It should instead check if CTRL is the only modifier pressed at the moment.