There are two public endpoints, all you require is a username to use.
https://duolingo-api-five.vercel.app/user/{username}
This returns the duolingo streak associated with the username and nothing else.
https://duolingo-api-five.vercel.app/user/svg/{username}
Returns an SVG with no background of the users current streak, as shown above.
You can add a snippet like this into your github readme to use the SVG, just put your duolingo username in {username}
<img src="https://duolingo-api-five.vercel.app/user/svg/{username}" alt="Duolingo Streak" width=360 style="border-radius:16px;">