I think it depends on what type of learning you prefer. The benefit of taking classes is that mistakes you make can be corrected by your teacher on the spot. I prefer self-study because I find there is too much distraction in a classroom full of people. The only thing I haven't had much training in is the speaking element, luckily there are platforms like iTalki to make up for that. Hope that answers your question. Best regards, Marijn

It probably also depends on how close your target language is to your own. If they are somewhat similar, you can probably have success teaching yourself. But if they are different then you might benefit from having a teacher there to correct your pronunciation and grammatical mistakes.

Sure, but it depends on the language. There are a lot of resources for learning English online so you can probably have a reasonable go at it yourself. It helps if you live in a place that speaks your target language. Myself, I didn't get much out of classes, I really had to put myself in a situation where I was forced to learn, so I moved and got a job in a kitchen where my target language was spoken, and that's how I found success in learning. Others really thrive in lessons, some pick things up just by playing video games (this actually works pretty well). There's tons of ways to learn languages, but typically lessons are a good way, and a teacher can even help you find resources so you can learn on your own. I've had students that just wanted a list of topics and some practice exercises and off they go. Everyone's different.

I think it is a personal thing. There is a lot of material online now. Do what works best for you. I suspect that listening to TV shows, radio and other sources will only get you so far. Eventually you probably have to interact with native speakers or teachers in order to become a natural speaker. I may be wrong. :)