The problem is, if you stand on an item in the resource bundle and then press 'new property'.
After you put the text into the key and value fields the new property will get the same value as the property you were
standing on.

Test setup:
Created one property file, with an additional language.

Steps to reproduce:
1) Click on a row
2) Click new property
3) Fill data in
4) Press OK
5) The value in the 'Default Language' column will be overwritten by the value of that property that you were last
selected. (What a long sentence :-)

If is no matter whether you clicked on the key column, the additional language's column, the default will be overridden.

Additional info: 
1) If you press 'remove property' the confirmation dialog will ask whether I would like to remove the old property. 
2) If you press ESC, you will get the original data back.

It seems, that the added column does is not set as the current editing object.