Auto translate mails in Apple's mail app

Solution 1:

You can create a Service that acts on a text and then you'll be able to select any text and see its translation. In order to do that, please install trans from here: Translate-Shell Github. Place the binary somewhere in the system, for example /usr/local/bin directory. Then You'll have to create an Automator Workflow as a "Quick Action" (before it was called a Service).

It'll look like that:

Then save it as translate. After that you'll be able to select any text, right click on it and select Services -> translate.

Here's how it works (translating your post into Polish language):