Fix: notify locale on closed merge request email

What does this MR do and why?

Reason

Right now the email template for closed-merge-request-email notification can only be sent in English.

Result

Replace hardcoded raw string with i18n translations.

/cc @prajnamas @orozot @qk44077907

Steps to verify

1. Pull this branch.
2. Start your GDK.
3. Go to gdk/rails/mailers/notify/closed_merge_request_email for verify the changes.

Screenshots

Before

After

Screenshots or screen recordings

These are strongly recommended to assist reviewers and reduce the time to merge your change.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.