This thread is resolved. Here is a description of the problem and solution.
Problem: When people made a successful payment, the visitor goes to the “Thank You” page thanks to the callback URL of Mollie (Payment Provider). The /fr/ is removed from the URL, and the callback breaks.
Solution: Updating WPML + WooCommerce & Mollie payment plugins solved the problem.
Several changes were coming with WooCommerce 3.6, for this is essential to test this first with the newest version.
Relevant Documentation: https://github.com/woocommerce/woocommerce/blob/master/CHANGELOG.txt
This is the technical support forum for WPML - the multilingual WordPress plugin.