Checking for duplicate emails in user_requirements() does not take langcode into account

Problem/Motivation

The users_field_data table has a langcode field, which allows translating users. When this is the case, the check in user_requirements() adds a warning to the status report page about conflicting user names, while this is technically the same user.

Steps to reproduce

1. Translate a user account in multiple language
2. Observe the warning on the status report page

Proposed resolution

When checking for conflicting user mails, we should probably take the langcode and UID into account as well.