Really, both are correct, although the second one is more natural. In the first, the meaning is 'I am intermediate (level)', and so intermediate is an adjective. In the second, you're using 'intermediate' as a noun. Hope that helps!

Yes Ian de Vries I definitely agree with you, but sometimes you have to introduce yourself just by texting without planning a conversation with your partner. It is this context that I ask for this question.

I am not a big fan of levels. I prefer, "Hello, my name is _________?" If you speak to a person then it will become evident your level during the conversation.

Dear Sir , You can use I am an Intermdiate in English or Myself an Intermediate in English.

The fore one should be fine. I don't think the latter it's correct.