Supplements for the Japanese Course
Seeing as much of the course is still in Beta and I'd reason it would take a long while assuming it's possible to become fluent with just DuoLingo, here are some books online and free to access that can help. (They are PDF's, just a heads up for those with limited space)
https://ia800201.us.archive.org/29/items/Genki/Genki%20-%20Elementary%20Japanese%20I.pdf - More of a traditional textbook (one some of you may recognize), goes over various things in Japanese. First few pages are in Japanese, switches to English afterwards.
http://www.guidetojapanese.org/grammar_guide.pdf - For those who don't understand the grammar, particles or the likes.
http://lukeranieri.com/japanese/files/Heisig-RememberingTheKanji_vol1.pdf - This covers a lot of Kanji and focuses on memorizing them without doing the traditional write it over and over and over again.
Best of luck to you all, hope these help for anyone having trouble!
Several other courses and apps were mentioned in the comments so I'll be adding them up here as more come in and have them divided by what's free, what's not and what has a premium paid option but is still helpful without a premium. Those I haven't tried and can't try I'll leave what other users mentioned about it underneath.
LingoDeer (App) - "If you want an app that is similar to DuoLingo but presents more grammar info, I would strongly recommend LingoDeer. It does a great job presenting new vocabulary and grammar concepts with clear explainations. And it is divided into bite-sized lessons just like DuoLingo. Very cute and polished little app." Katherine97727 LingoDeer also has lessons on Chinese and Korean. EDIT; LingoDeer now offers Vietnamese, Spanish, French, German, and Portugese.
http://www.linguti.com/ (Website) - Linguti similarly teaches several other languages. It's entirely free and has a sort of game-like system. The website also has a language partner system
https://www.wanikani.com/ (Website) - "Personally I've found WaniKani the most useful, to the point that it's now my main resource. It's for Kanji and vocabularly, and after a few months, I'm starting to actually know much of the vocab here on Duo. I also think it's probably more applicable to real life Japanese (because most is written in Kanji). It's a fun SRS app, kinda Duolingo esque." - John-JoseN "And I can second WaniKani, I have almost entirely switched over there. Although their primary focus is on learning Kanji so you'll have to look mostly elsewhere for the grammar. The community is very active though." - sn2112
https://www.japanesepod101.com/index.php (Website) - This is a Japanese Podcast. I'm listing it under the buy section though it does have some free options, I don't suggest using it without a sub. Most of their podcasts are walled behind a pay option. It's a good option if you can afford the monthly payments. In addition, they have several other versions of this Podcast in other languages.
https://www.memrise.com/courses/english/japanese-4/ (Website/App) - There's a website version and an app version of this. It's customizable in the fact you can create your own courses and there are pre-made courses available. Both by other people and the official Memrise team. You can only add unofficial courses to your list through the website. One important note, the official courses are good and the app is available on both Apple and Android but you must have premium if you use the app on an Apple. Unofficial courses work on both Apple and Android meanwhile. Memrise also teaches many other languages.
Personally I've found WaniKani the most useful, to the point that it's now my main resource. It's for Kanji and vocabularly, and after a few months, I'm starting to actually know much of the vocab here on Duo.
I also think it's probably more applicable to real life Japanese (because most is written in Kanji). It's a fun SRS app, kinda Duolingo esque.
Free for first three levels but then paid. (No financial conflicts for me, just a happy customer).
It was actually linked to me by a good friend of mine also learning Japanese. So thanks to him for that.
I may update/edit then and add WaniKani to the list. Or turn it into a MasterList of resources, books and everything else. :) May I ask if there's anything you personally recommend?
memrise.com is another good website for words. I am making my own personal course on it so it's customizable, like Tinycards. And speaking of tinycards, I've heard the website Anki is much like it, though I haven't used it myself so I can't say how it works. (More of a pen and paper studier, personally) As for grammar, Bunpro.jp highlights a lot of good grammar points and also has an SRS review system. Useful for drilling those points into your memory. Those are the big ones I can think of.
If you want an app that is similar to DuoLingo but presents more grammar info, I would strongly recommend LingoDeer.
It does a great job presenting new vocabulary and grammar concepts with clear explainations. And it is divided into bite-sized lessons just like DuoLingo. Very cute and polished little app.