Fix: locale on new mention in issue mail

What does this MR do and why?

Reason

The current version of "being mentioned in the issue email" is not localized, this MR is meant to fix the untranslated string in the email template.

/cc @prajnamas @orozot @qk44077907

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

1. Boot your GDK
2. Go to any project > issues > create an issue
3. Mention any user's name other than the current login user.
4. Go to gdk_host/rails/letter_opener to see the email.
5. Observe.

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

MR acceptance checklist

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