I've studied the article you send, and I had a few questions:
1: Every front-end field on our website has to be translated. Is it correct that every ACF field setting has to be "Translate"?
2: Is it possible to set all of the ACF fields from the default "Don't translate" to "Translate" in one action?
3: All the translation settings of the ACF fields are now "Don't translate". But on our website everything is translated in all 4 languages. How is this possible?
We set all the custom fields to translate and updates all of the WPML plugins + WP core to 5.2. The issue is not showing up this time. The only question we have is question 3 in my reply above. Nevertheless the issue is resolved!
My issue is resolved now. Thanks for your effort and your time!