Steps to reproduce the issue

1. Install a multilanguage site
2. Create two articles in different languages
3. Associate these in the backend
4. Edit one of them in the frontend
5. Click Save -> An error message is displayed and the article is not saved.

Expected result

Article saved

Actual result

The message "Field required: Category" is displayed and the article is not saved.

System information (as much as possible)

Additional comments

The cause of this is around line 380 in administrator/components/com_content/models/article.php where certain form properties are altered when editing an article with associations. Among these, the filter "unset" is applied to the catid field, which means the form validation will fail when the article is about to be saved by the controller since the catid field is also a required field.