In my experience, this is the best start in learning a language: learn to hear well, replenish vocabulary, and remember frequently used phrases. Grammar is then very easily understood and assimilated. Better to watch with subtitles in the native language.

It is a very good supplement in my opinion. Especially with Japanese and Chinese television shows you usually have everything subtitled in the language, so you can practice reading at the same time too. It isn't enough as the only way to study, but after you get to an intermediate level, you benefit more from them.

It can help but to learn a language by watching movies only is impossible in my opinion.

Zeynep, my friend from Tunisia learned Turkish this way. But she likes this langauge, there is a lot of Arabic words in Turkish and she knew some Turkish.

What she did is she just watched series without subtitles and at some moment discovered that she can understand everything.

I definitely agree that watching movies is one of many ways to learn a language. But of course, that is, if your goal is to improve your listening and vocabulary. If you want to practice speaking of course you would need someone to converse with.