Let's say you know 100% of the Dutch Duolingo course. At which language level would you be?
DuoLingo's goal is to make you achieve level A2, no more. Besides, you have to keep in mind that there's a lot of things you don't practice in Duolingo (speaking, conversation, writing...), so I would totally say that you cannot have a better level than A2 if you used only Duo.
Of course, if you practice Dutch by any other mean (discussing with a Dutch friend, reading Dutch newspaper, seeking grammar/vocabulary/pronunciation courses...) there's a chance you get to level B1.
Ik ben het helemaal eens met AmineHadji1. Ten hoogste A2 voor schrijven en lager in het spreken.
(I totally agree with AmineHadji1. At most A2 in writing and less in speaking).
Maar, probeer het zelf eens uit met behulp van deze gratis cursus van de universiteit Groningen.
(But, try it yourself by doing this free course of the university of Groningen)
Info course: http://www.rug.nl/society-business/language-centre/language-courses-and-communication-training/dutch-for-non-native-speakers/online/introduction-to-dutch-mooc?lang=en
Maybe you can test it yourself? Google 'Dutch A1 test' gave a few sites with free tests today.
Test wether you have 0, A1, A2 or B1+ level (on website of our Summer School):
Level determination test (takes 30-60 minutes)
I'll take a different angle on this. First off, I've finished 3 of the courses but don't know 100% of the course. Meaning that I don't use the 'hover over' reveal with my mouse and I still get some of the answers wrong. I recently took the Spanish and Italian proficiency quizzes and scored 2.5 and 1.8 respectively. My guess is that puts me at about medium (Spanish) and low (Italian) intermediate reading comprehension.
Having said all that, I'm still learning, getting better. I think ultimately by continuing to practice and challenge myself and using other resources I plan on getting my proficiency quiz scores as close to the perfect 5.0 as possible. Even then that might put me at close to Expert level reading comprehension but I won't get to thinking and speaking the languages at that level without the other resources.
Of course, even if you finish the whole course - the language is like a skill, and you need to practice in order to be good in it, otherwise you'll forget it. And I am aware of the fact that having Duolingo as the only source of learning a language is not enough. With Dutch you would probably get an even lower score, because there are less lessons for now.
I hope to seek certifications in a number of the languages I'm studying on Duolingo, though I face rather significant barriers to being able to even attempt the exams. It will be difficult for me to marshal the resources for further study materials and contact with active speakers in the New York NY area. The most I'm likely to be able to do is once a week or less social gatherings at bars dedicated to practicing some of the various languages I'm studying. The exceptions are one-hour free weekly events for French and Italian and enough Spanish speakers about that I likely don't need to make special arrangements. Given that, I don't expect to be able to achieve C1 in anything but Spanish, which is more of a question of regaining lost childhood fluency than new learning. I might be able to boost my German to B1 or maybe even B2, but that largely on the basis of regaining lost proficiency from having pursued a minor in it at university.
What little I know of the exams is that the vocabularies they demand are far larger than Duolingo covers, and the speaking and listening proficiency demands are far beyond anything the courses exercise. Judging extemporaneously produced responses to speech in the language for clarity, fluency, flexibility, organization, and well-structuredness is far beyond Duolingo's basic speech recognition AI, if it can even be called that. The descriptions of the CEFR language proficiency ratings at https://en.wikipedia.org/wiki/Common_European_Framework_of_Reference_for_Languages#Common_reference_levels seem to suggest Duolingo would be adequate preparation for A1 or A2 at most from my impression of it.
Wow, you're such a polyglot! I'm impressed by the number of languages you're learning :) Do you do it for your own pleasure or do you intend on doing something with them?
I would say that Duolingo definitely prepares you for A1 and if you do a whole course, then A2... I would also say that combined with some practice from the real-life, it might even go up to B1. For me it's really debatable whether you can actually go to B2 and up (that is, only for the Dutch language), and that's why I'm asking for different opinions. :)
I'm trying to eventually get certifications in as many of the TELC languages as I can, cf. https://en.wikipedia.org/wiki/The_European_Language_Certificates . I would probably have to travel to the Netherlands or a territory thereof for certification examinations in Dutch, which I am studying, since it's not one of the TELC languages. I would probably have to bootstrap to the point where I could travel for interaction in Dutch anyway, so it's still reasonable to take it on with the expectation of only gaining certification in it after departing the US, all of which comments also apply to Swedish.
Sadly, few of the languages available for English speakers are pertinent to the Fulbright or Boren scholarships. Turkish may be the only complete course for one, and Vietnamese and Swahili may count once speaking drills are developed for them, if they ever are.
It may be redundant, but I would say that nothing above A2 could be achieved without a very substantial amount of learning materials other than Duolingo on the basis of vocabulary alone, and with other factors related to speaking and listening reinforcing that limit.