“being into something” is a rather informal phrase often used by the young, not something that you’d often hear about a language. I’m into Marvel movies/French girls/jazz. I’m passionate about learning new languages. I adore the English/French language.

No... "I'm in English." is something you might say to your friend who calls you on your mobile phone, in the middle of your school English lesson. Although, in this situation, it might be a good idea to text this message, in case your teacher hears you talking. 🙂

How about in?

Yeah that's correct, "to be into (something)" is a phrasal verb. Phrasal verbs don't necessarily follow logic 100%, so you have to memorize them.

Hi Shaghayegh, “to be into” something is a phrasal verb for talking about your interests. Some people think of it as casual, but it is a very common expression. But “in” is just the ordinary preposition that we use when something is located inside a space with edges or borders, or during a period of time that starts and ends. “English”, the language, doesn’t have edges or borders really. So “I’m in English” doesn’t make sense. But “I’m in English class” does.