Fix untranslated form button values

I still believe that these should be put back to where they were for a better user experience for novices as they will see e.g. "COM_FABRIK_FIELD_SUBMIT_BUTTON_LABEL_DEFAULT" in the Save button text field instead of "Save".

My intent with the original change was to set the default text to "Save" translated into local language, not to put a COM_FABRIK string in there, however Joomla does not translate the default value.