Once you save the options for your site’s default language or a custom language, it is not possible to change their language code.

If you want to use a different code for a specific language, for example, jp instead of ja for Japanese, or if you want to change the language code for a specific language you have to do following:

  1. Add or create the language you need.
  2. Bulk duplicate the content to the new language from the WPML -> Translation Management page.

If needed you can also:

  1. Export and import .po files from the WPML -> String Translation page.
  2. Delete the old content and the old language.