DeepL languages request missing languages

Problem

The DeepL languages request returns different response where some of the available target languages are not listed (EN-US, EN-GB etc..) based on if the request has the
Content-Type header set to 'application/json'. When the content type header is removed for this request, deepl returns the correct languages list.
 
Warning: after changing this, as target language no longer will be available the selection of the major language (ex: en will be no longer available, we will only have en-us or en-gb) we should validate if this will produces problems with already created resources (where the target language is en for example. ).