Steps to reproduce:
1 - Create a content type
2 - Create some objects using this new content type, and in multiple translations.
3 - Now edit the content type and add a new required field and save.
4 - Try to edit one of the previously created content objects using this content type. The result will be you cannot save and the error says there is a missing field in the other translation. Switch to the other translation and fill in the missing field, try to press save again and the same error but for the other translation you just filled out.

This is a complete block. The field added is required, the editor cannot make changes to the content object...

The only work around is to change the content type to not required but this then breaks things elsewhere.