I believe that self-study is essential to success in any language - any field for that matter - and that one won't benefit from a teacher without doing significant work ALONE first.

I usually begin studying a language by listening often and reading as much as I can. When I've developed a "feel" for the language, I practice reading aloud, making videos talking about simple everyday things, and building a set of practical productive skills at each level, A1 through C2.

Teachers are useful to me for motivation, for confidence speaking, and for immediate feedback and clarifications. Keep in mind though, I'm a bit of a lone wolf and typically like working alone. Some people like learning in social situations, and need more guidance, more frequently. A lot depends on your character, work habits, and experience as a language student.

Yes, that is quite possible especially nowadays many learners can approach listening, speaking, reading and writing via social networks, applications, Youtube...etc.  In comparison with English learning 30 years ago, you might have had to rely heavily on face to face speaking and listening. 

Of course we all have to admit if one has an intimate friend like couples , one may have a better position to learn that language with more efficiency, but in the real world not everyone can have such a chance to learn a language and even so, it entails industrious learning to achieve reading and writing.  

Hi Sepidah, I firmly believe in the importance of learning to be your own teacher - learning to fish, rather than being given fish. 

Some people say, and I agree, that each language you learn becomes easier. I think it's because you learn the principles of the process, and the path becomes more familiar. So learning the principles of language learning could be as important as learning any one language, if learning multiple languages is the goal.