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Here are the steps to reproduce the bug: 1) Create a new Java Application project 2) Add a JPanel 3) Select "Automatic Internationalization" for the form 4) Add a JButton to the form 5) Click on the "..." button of the form property "Design Locale" (value should be "default language" now) 6) Add a new language (I selected Language Code "de" and pressed OK) 7) Notice that the value of the "Design Locale" property jumps to "_de", which is already buggy (it should be "de"). 8) Try changing the value of the "Design Locale" combobox. It only shows "default language" although I just added another language. This way I can not translate the program into another language. The problems in (7) and (8) are probably two symptoms of the same bug. Changing priority to P1 because without working support for I18N Netbeans is absolutely useless for me...
For some reason, locale created this way is not properly recognized as part of the existing properties bundle (separate file appears in explorer). As a workaround you can create the locale directly on the properties file in explorer, then select it in the form. The separate entry also gets fixed and can be used normally after IDE restart.
Fixed as part of issue 148556. http://hg.netbeans.org/main/rev/112c770b62d5