In short, the x-default will always be assigned to the default language set within WPML->Languages. It can be changed but that requires custom work which is outside the scope of our support.
Where can i see to modify the x-default using custom code? The problem that i encounter is that we are using 2 websites which is set as per domain language, and each website has its own wpml installed.
The x-default will always be the default language select if you want to change it you can do it using this code from the following ticket, but do note this is not an official suggestion and simply a workaround, if further adjustments are required we recommend reaching out to a developer or a WPML contractor from here : https://wpml.org/contractors/
