Adding a new required field to the Content Type with Translation will break editing on previously created Contents.

Steps to reproduce
1. Clean installation of eZ Platform 2.5.5
2. Log in to the Back Office as a admin user.
3. Add a new Language: German | ger-DE.
4. Create a new Content Type Test Content with only one Field: Text Line Name, required.
5. Create a new Test Content Content. Name it Test Eng and Publish it.
6. Add a new Translation to Test Eng: German with name Test Ger and Publish it.
7. Edit Test Content and add a new Text Line checking Required checkbox.
8. Go to Test Content. Edit it in English.

Result
Content Edit view will not open and at the bottom of the site red bar with "Content fields did not validate" will be shown.

Expected Result
User should be able to edit Content normally.