Can you become fluent in esperanto with duolingo?
If someone were to complete the Esperanto course what would their fluency be? Esperanto is relatively easy so it should be a high fluency im guessing
Short answer: No
Longer answer: Duolingo will introduce you to the language and hopefully peak your interest and through that you'll reach out and find other resources and possibly wind up finding yourself singing weird Esperanto songs to yourself in the car and reading books in Esperanto and then maybe find an increasing interest in the Esperanta Movado and somewhere in that journey, if you're not careful, you may become something close to fluent.
In Esperanto, unlike in other languages, it doesn't take much time to internalise the whole grammatical structure of the language, and thereafter it's just a matter of acquiring vocabulary.
So I think if you compare learners who have completed the Esperanto tree with those who have completed other trees, the Esperantists will be closer to true mastery of the language, even if their vocabulary is no greater.
Generally you can't speak a language fluently if you've only learnt the language by doing multiple choice quizzes and translation exercises (i.e. the Duolingo method) - though those obviously help.
If you use other means, such as watching videos on YouTube, going to events (e.g. at your local club, or Duolingo events, or even international events), practising speaking through Ekparolu, reading books/magazines/blogs/etc., chatting online (e.g. on FB/VK/Telegram/etc.) then you can certainly use the Duolingo course as a basis to developing fluency. It does provide a good spread of grammar and vocabulary.
I fully agree with lectroidmarc's short answer: No, Duolingo will not make you fluent in Esperanto.
Here's my longer answer:
At the risk of sounding stodgy to all the 16-year-olds reading along, back in my day things were better. We walked up hill both ways to school. And if we were REALLY good, we didn't get dessert. Oh yeah, and we actually LEARNED Esperanto.
Yes, I've seen people who say they've learned Esperanto on Duolingo and they speak it quite well. I've also seen people claim to use only Duolingo, but somehow fail to mention that they'd taken a dozen private lessons with me. In some cases - such as the guy who claimed to have received a B2 certificate in German using no other materials than what he could find on Duolingo - I'm inclined to believe the stories - but these seem to be the exception.
When I was in my "language addict" phase, I was greatly influenced by Barry Farber's book How To Learn Any Language. One of the bits of advice in that book was not to spend a lot of time worrying about which course to do. Go to the bookstore and buy one of everything. When not taken too literally, this is actually very good advice. Do not limit yourself to one course - especially if that course is Duolingo. Duolingo has its place, but it's especially good for spelling practice and vocabulary building. Even if you read the tips and notes (which many people don't do), it's not the best way to learn grammar.
Within a month of starting Esperanto, I was using it with real people. It was the only choice back then - well, that or books. Within a few months, I went to my first event. Within a year, I had stayed in the homes of people with whom I had no other common language other than Esperanto.
With that in mind, it kills me when I see people still plodding away here on Duolingo after three years. I think at times it's too tempting to keep doing the same exercises over and over because they're familiar. The only way to become fluent is to get out there and use Esperanto. Another issue with the Duolingo course is that beyond a few paragraphs here or there, there's no connection with the history or community.
This is why I think Duolingo is great - if you're combining it with another book, course, or private tutor.
Here's a playlist of videos that I made just about a year ago about how I became fluent in Esperanto, and what I would do if I were starting over now.
"Language Addict" link https://youtu.be/PoQryJDs_bs?t=116
What Duolingo is now, was, and was ever meant to be isn't exactly clear to me. "Learn a language in 5 minutes a day" seems to mean "learn to speak a language."
This intro from Duolingo themselves gives a very different impression of what Duolingo is (was) for.
The promise seems to have been for high quality translations from people who are learning and progressing from Duolingo alone. Ultimately Duolingo is what it is and it's up to us - the users - to figure out how to make the most of it - but if the question is what Duolingo was designed for, I think that's a different question.
I don't think you can be fluent at any language just with Duolingo or just with any course if you don't practice that language with people in the course or out of the course.
But, in the practice part Esperanto is easier to use than other languages, so you probably will be more fluent than in other languages.