When you edit an already published Web Content, and change the default language, then this language is still there on the "Add Translation" dropdown list.
1.) Go to Site Administration --> Content --> Web Content, here create a new Web Content.
2.) Edit the Web Content and change the Default language to German.
3.) Click on the "Add Translation" button.
Result: German is still listed on the dropdown list.
Expected Result: Because German is the new default language it should not be possible to add German as a translation.