I'm not sure if it's really a bug, but I couldn't find any logical reason for such protgram behaviour.
When I add another language (in Languages section), a new check box apears in the "Language" tab for the Files and Dirs items.
The problem is that the check box list is disabled (so I can't select any language). Only the text field at the bottom is enabled. If I type for exmple en to the text field, the click OK, and open the Properties->Language tab again, the check box list will now be enabled, and the text field disabled (which I suppose is the normal way it should work).
It seems to work in the "wrong" way (check box list disabled and text box enabled) when the item does not have "Languages" parameter.