It's not possible to change main language of a Content Item because of strict types.

Setup:
1. Add a new Language: Polski (pol-PL) in admin panel
2. Set up admin_group to use pol-PL lanuage (in ezplatform.yaml)

Steps to reproduce:
1. Login as admin
2. Go to Content -> Content Structure
3. Create a new Folder and publish it
4. Translate it into polish language
5. Switch to "Translations" tab and select polish translation as the main one

Expected:
Main translation is changed

Actual:
Exception:

Argument 1 passed to eZ\Publish\Core\Repository\SiteAccessAware\ContentService::loadContent() must be of the type int, string given, called in /Users/mareknocon/Desktop/Sites/v3/vendor/ezsystems/ezplatform-admin-ui/src/lib/Form/DataTransformer/ContentTransformer.php on line 70