Problem:
The client wanted to remove his primary language. Unfortunately you can't delete the primary language of your site.
Solution:
First you need to make your secondary language the primary language and then remove your old primary language.
- Make a full back up for your site and database
- Go to "WPML > Languages > Site Languages" and click on “Change default language”, select select your secondary language and Save.
- After that you can remove your old primary language (that became secondary) completely with the "Add / Remove languages" button.