If a language is not available in the release try to see in the download Translations section of the site.

To install the language file Place it in the PlugIns folder inside the application.

Linux:
User_folder/voixmanager/PlugIns

Windows:
programs_folder/Voix Manager/PlugIns

MacOSX:
make  "ctrl click" on the application  and select  "show content" then  place the translation  in the  PlugIns folder inside the  Voix Manager application package.