In the last releases of GanttProject (starting from GP 2.0.7) Polish translation was broken . Choosing "Polski" in the language settings didn't work and even manual editing of the options file didn't help. The reason was one missing character in the translation file.
The fix was submitted to our SVN repository on Google Code Hosting and will be included into GP 2.0.10. If you don't want to wait until 2.0.10, just open
in any text editor ($GP_HOME is a directory where GanttProject is installed on your machine), find
and append 'c' at the end of that line, so that it becomes