For a classical user (admin for example), if you try to translate the content in another language, the "ezuser" attribute is not translatable, but form validation fails with the following error :
Required data is either missing or is invalid:

• User account: Input required.

With this validation bug, it's impossible to translate the user content.

Steps to reproduce

In admin interface, go to Administrator User content, and edit it in another language (initial language is the english for example, so edit it in... French ?).
Complete all attributes ("User Account" attribute is not translatable, so you cannot edit it...)

Try to validate edit form : the validation error occurs :
Required data is either missing or is invalid:

• User account: Input required.