I did Duolingo seriously for more than 370 days in a row. Spanish: I speak french and English so Spanish is relatively easy to complete. After completing the course, I tried having conversations with people (with friends but mostly uber drivers) and I was surprised how many times i learned it all wrong. I had to read a lot or children book to remedy that. French: As a french speaker, I went through it as a meta cours…
Well, if you're coming from a European language background, Japanese is one of the hardest languages to benchmark on. I mean, there's three writing systems (hiragana, katakana, kanji) for starters! And then unlike European languages, it's not alphabetical and kanji is not even really syllabic - which means, the background you need to get to be able to read texts is huge.
E.g., for Spanish, I can read the newspaper (not understand yet, but just reproduce, look for words I recognize, etc.) without even touching a language course. And if I get the basics, I can see grammatical structure, and given common Romance background, get about 20% of vocabulary for free just from knowing another European language (well, not true for all European ones, but for many it is).