All plain text, each carrying the link that makes it actionable:

a receipt when someone writes to you;
a note to you when a request arrives or a customer replies, with their message in it, and to whoever answers the request's topic when it arrives. A new request about a product names the product, with a link to its page;
a note to a colleague when a request is assigned to them;
once a request is marked urgent, the notes to your team about it (the customer's replies, and a request assigned to a colleague) start their subject with [Urgent]. Emails to customers never mention a priority;
a note to them when you reply, with your reply in it;
a note when you mark something resolved, which asks How did we do? while ratings are on;
when a request resolves itself because the customer did not answer, the same note saying so, and, if you choose, a reminder a few days before (see When a customer does not answer under the support desk).

The receipt does not repeat the customer's message. Anyone can fill in the form with any address, and a receipt quoting their text would have your site deliver it for them. Private notes are never in any email.

Emails to customers are written in your site's language, even when you reply from an admin set to another language. Assigned to you is written in the language the colleague chose in their own profile, or your site's language if they left it at the site default.

Notifications to you carry the customer's address as the reply-to, so hitting reply in your mail client reaches the person rather than a support inbox nobody reads. Routine status changes send nothing — telling someone their request moved from "new" to "open" is telling them nothing.