I don't know. But ... my Mac computers don't ever offer to translate while pasting for me. Not with DU, nor with any other site.
But since nobody else has replied, I'll have a guess.
I'm going to guess that the translation service you see is ...
(1) offered as a service by your OS and/or browser
(2) not really anything that's being done by DU
As to your observation, I'm going to wildly speculate that the browser is looking at your paste buffer and if it thinks it might be useful to translate (i.e. it doesn't appear to be English when sending to an English web site), only then will it offer to translate.
But, what OS are you using? What browser? On my machines, I can select a buffer (in a drafts window) and it'll offer to translate it for me. But not directly while I "paste" into a composition window.
In any event, the following page did a decent job of describing the translation abilities of different browsers. Maybe it'll help clear things up for you:
https://lifehacker.com/tech/how-to-quickly-translate-text-in-any-web-browser
Increasingly, web browsers will automatically detect a website written in a language that's not English, and will either instantly translate the pages or throw up a dialog box asking if that's what you want to do, but there are a few more customization options available if you dig deeper into the tools.
Here's how to find the translation features in the biggest browsersChrome, Edge, Firefox, and Safarion both desktop and on mobile.
...