This thread is resolved. Here is a description of the problem and solution.
Problem: The client is using the domain per language functionality and he has 2 languages hidden at the moment by using the WPML settings for this. He said that the domains for the hidden languages (which are currently in development) are serving the old site and he wondered if he can somehow use a language attribute for the hidden languages.
Solution: When you are using the domain per language functionality, you can't use any other option like the language attributes (eg: ?lang=en). After the client marked the languages as public (not hidden anymore), things started to work normally again.
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