Internationalisation of diff_note directory

What does this MR do?

i18n the diff_note directory

This is one of many MRs opened in order to improve the overall internationalisation of the GitLab codebase.

https://docs.gitlab.com/ee/development/i18n/externalization.html

Related issue is https://gitlab.com/gitlab-org/gitlab-ce/issues/59733

EE Port: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/11728