It is more than 5 hours and I am still searching for a solution.
I have found how to remove "language neutral" from language list of multilingual node-forms. But english is always selected now. It doesnt matter if current language is english or german or french...
I want that Drupal selects current language on node/add forms with language prefix.
example.com/de/node/add --> german must be selected
example.com/fr/node/add --> french must be selected
example.com/node/add --> english