From OXIDwiki

Please read this article  if you are not familiar with language files handling in eshop. 

Now when you are ready to edit language files please follow these instructions:

• find out active theme name - go to admin > Master Settings > Themes;
• using (S)FTP client open customizable theme language file located at e.g. "/out/azure/en/cust_lang.php";
• add new constant(s) which format is e.g.:
  
  '_UNIT_KG' => "kg",
  
• save modified language file;
• cleanup "/tmp" folder contents.