This thread is resolved. Here is a description of the problem and solution.
Problem: Client want the site to optimize for English Speakers (browser language), Google in English, no matter if they are in US, UK, AU, etc. The same for Dutch. No matter if they are in Netherlands, or elsewhere.
Solution: WPML downloads automatically .mo WordPress translations files any time you add a new language. For this, it needs to match the Language tags with the ones provided by WordPress.
When you download the dutch wordpress, inside wp-content/languages you will find the default admin language files. You can copy them to your own wordpress and rename them to skip the regional code and then change the language tag in WPML > Languages or change the output of the hreflang tags with code.
This is the technical support forum for WPML - the multilingual WordPress plugin.
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