Fix: notify locale on approved merge request email

What does this MR do and why?

Reason

Right now the email template for approved merge request notification can only be sent in English

Result

in this MR I updated the template with i18n enabled strings instead of hardcoded strings. After this MR it will be able to send languages the user prefers.

/cc @prajnamas @orozot @qk44077907

How to test locally

1. Start your GDK instance locally or via gitpod or other ways
2. Open the browser with your instance with path /rails/mailers
3. Find the email template with cmd/ctrl + f search for text approved_merge_request
4. Open that link to see the previews.

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