This thread is resolved. Here is a description of the problem and solution.
Problem: When the client tried to send content to his translation service, he got the message "This service does not support translation from nl"
Solution: Please go to WPML -> Translation Management -> Translation Service and click on "Refresh language pairs" button and then try again.
Relevant Documentation: https://wpml.org/translation-service/
This is the technical support forum for WPML - the multilingual WordPress plugin.
Everyone can read, but only WPML clients can post here. WPML team is replying on the forum 6 days per week, 22 hours per day.