When I started this post, I had finished the French Duolingo course.
Then I saw that I was making good progress on the Italian one, so I finished that as well.
Then I decided to finish the Spanish course, and made my goal Easter because it's a public date. I had done the same thing with Japanese and Portuguese last year; I finished Easter night. This year Easter was Sunday, and I finished the Spanish by Thursday. By that time, I was so tired from doing all those lessons and tests in my free time, I relaxed for a bit.
This is what it looks like when you reach the end: the owl is reading, and when you finish the Daily Refresh, the owl cheers (as pictured above).
But the app doesn't. Duolingo doesn't have any congratulatory message when you finish because I suspect they don't want you to, which is why I attempted to go past the finish line. What was frustrating was that for the past couple of years, I was making progress on a language course, then Duolingo would add more lessons, extending the paths. Then when I made more progress, they'd add more. Since they kept changing the goalposts, I decided to change mine and play to win.
At this point, all six languages I've finished are on Daily Refresh, and it resets every day. I decided to stick with German until my subscription ends; I'm going to Germany in late summer, so I really need to improve. I was so focused on finishing that I neglected other German sources; now I will spend more time there before my trip.
Here's what it looks like when you reach the end of Japanese, Portuguese, Spanish, Italian, French, and German. The gold color is when you do the extra challenges at the end, but they're all on Daily Refresh at this point. Remember: Duolingo probably doesn't want you to finish, but don't let it discourage you from trying.