Usually the best resources you can get online for free for language learning as a beginner or intermediate is mostly pirated photoscanned books, with some online pages or YouTubers mixed in as another supplement.

For example, for Japanese, I pirated the 3rd editions of Genki 1 & Genki 2, plus the workbooks for them. I also owe a lot to the YouTuber Tokini Andy (who has videos going over the updated Genki textbooks and explains a lot of things missing/poorly explained in the book, he's pretty great). There's also an interactive quizzing website for Genki someone made. For definitions and stroke order I used jisho.org, and for etymology I used Wiktionary, hanziyuan.net, and dong-chinese.com. The only other website/app I regularly used was Renshuu (a spaced repitition learning app for Japanese), which was pretty great and was a good supplement for drilling in what you learned from other resources – although you have to modify the settings quite a bit to really "optimize" it. A lot of people use WaniKani which is kind of similar but I think more Kanji-oriented and used more for students studying before a test. Oh and to learn the Kana, I basically just tested myself writing down the characters in the standard order for a few days until it got drilled into my head, I also used the Tofugu Hiragana learning resource thing but it only helped for a few mneumonics. Other than that, although I'm not exactly a weeb, I tried to force myself to watch anime (in Japanese) any time I wanted to do something like watch a show or play a game, and I'd look up all the stuff I didn't recognize. Anime isn't exactly representative of how Japanese people speak at all, and you're going to get your shit kicked in (by that I mean disapprovingly stared at) by Japanese people if you speak like an anime character, but I suppose it's like learning English off of Sesame Street and SkyDoesMinecraft...

For German, I heavily utilized Deutsche Welle's learning resources, especially Nicos Weg. I also had people to practice the language with. My German still sucks though... for whatever reason, I had the absolute most difficult time with trying to learning German out of any language. The word order magic fucked with my head especially, but I just kept mixing up basic words.

For Russian I used Memrise at first, which worked for vocabulary and got me familiar with the most very basic vocabulary, but the features locked behind monetization eventually got too disruptive so I spent pretty much all my time on (pirated) beginner Russian learning textbooks and very technical grammar books (I was a very learned linguistics major so learning from linguistics-heavy books was significantly more feasible for me than it is for the average person). I probably had the least frustrating time with Russian out of any of the languages I self-studied. I self-taught myself Cyrillic when I was like 8 because I thought slavic stuff was cool so that didn't really require any time...

I learned passable French in high school (despite my ADHD ass not paying attention 99% of the time and basically just not being present mentally for all of French 3), which then degraded a lot in my ability to use it since I never used it and was preoccupied with other stuff, but I can still read it fine, and I can understand it spoken depending on how they speak and my state of mind. I didn't even study outside of school or anything really, I just had a teacher from France (she was my favorite teacher). Actually thinking of the words and grammar I'm trying to say though, I'm pretty fucked in that department unless I go back and practice it. French is my 2nd language.

I self-studied Spanish after school, not very seriously though, I could already grasp it pretty well enough because of my French knowledge. I got conversational in no time – still, randomly not being able to recall random words is a pain in the ass (that goes with English too I guess). I did a bit of Duolingo at first but then just started listening to podcasts and videos and stuff, and looked up the words I didn't know (beforehand and during the time I was already doing a lot of Spanish linguistics work so I already knew "about" the language and its phonology/spelling to pick out the things I heard). This was really only possible because, again, I was already able to understand pretty much all the French you would encounter in daily life.

For Finnish I mostly used pirated books and YouTube learning resources. The first books I used were from Leila White, "From Start to Finnish" and "A Grammar Book of Finnish" which are both great. I would definitely suggest that anyone who wants to learn Finnish goes through those books – the second one's a reference rather than something you're supposed to go through from beginning to end though.

I tried (and failed) to learn Arabic a reeeally long time ago. It didn't extend much past a few obscure and not-very-helpful learning internet resources plus Duolingo (which was kind of useless for Arabic, even moreso than Duolingo typically is useless for languages). I had an unusually hard time with the script for this one (is it racist to say the damn squiggly lines all look the same), and reading it without vowel markers is very difficult to me.

slovake.eu was a helpful resource for Slovak when I was doing the necessary stuff for citizenship – I could've probably gotten by with only English (despite doing it myself and without an immigration lawyer) but I felt it only made sense to at least try to learn the language. It... definitely happened... I basically learned much of the core vocabulary and some grammar quirks, and played fill in the blank with Russian/Polish words except how I thought they'd be in Slovak for words for ones I didn't know. Would not recommend doing that, but if you DO do it then just know that Slovak is the absolute best slavic language to do it in, they will probably understand you if you outright are speaking a different slavic language.

Right now I'm using this book online for Italian that's only in Italian which is basically like, introducing grammar & topics in the language with no actual instruction or anything, it's just a bunch of Italian sentences with images and stuff to get you to remember the grammar based off of context. It's sick as hell, but I can't remember what it's called right now.

After a long time (around B2 or maybe B1 level probably) you have enough comprehension to start learning well while watching content made for natives – e.g. you can watch YouTube videos or a TV show in the language and can learn from looking up the (still large) portion of words you don't understand.

I was originally a monolingual English speaker – only 1 native language, I didn't have like 2 or 3 native languages like most of the world (shout out to all the kids who acquired English solely off of TV and YouTube as a kid). On one hand, being monolingual definitely makes you more ignorant to other languages and your first language might be a little bit harder (but honestly it doesn't get much easier from there, you will still writhe in pain 4 hours a day trying to learn any subsequent languages), but on the other hand being a monolingual ENGLISH speaker opens you up to way more possibilities (resources) than not being an English speaker, so I guess overall I was pretty lucky in that regard.

I have really bad ADHD and Aphantasia which is (for the most part) a hinderance to language learning – my working memory is extremely bad, I can't visualize SHIT and ADHD makes my non-visual memory go kaput. Language learning takes significant time, energy, it's frustrating as fuck, and most of this comes down to a lot of it just being brute forcing memory. There is no cheat to language learning, there is no "lern basque in 23 dayz", it is just putting thousands upon thousands of hours of very regular, very (inter)active focus and memorization methods into it until it's drilled in your head. It can be more challenging than any job you've ever done and you'll want to cry due to how little progress you feel that you're making despite the great amounts of time and energy you put into deciphering this mess.

On the other hand, I know Autistic people with Hyperphantasia and Synthesia (specifically, the kind where you see colorful words appear in your vision when you hear, read, or think about language) and they are SIGNIFICANTLY more capable at language stuff than anyone else, although it's still a lot of effort to put in for them of course.

Language is pretty much about constantly pushing back against the force inside of you that says "you can't do this, you're not improving, it's tiring" and managing your breaks/scheduling well. It's easy to turn a "break" into just not touching the language again. You have to be motivated a LOT to learn a language (including being motivated by pressure/necessity, go get locked up in a country with speakers of your TL or something) or else you'll just quit as soon as you hit your first wave of roadblocks.