Neither is correct You can say I know how difficult it is to learn another language. Or I know how difficult learning another language is. How in this sentence changes the way the words act a bit. Usually difficult takes to: It is difficult to play the piano BUT NOT It is difficult playing the piano. but in this case How means we need to use difficult it is to + verb OR how difficult verb-ing is. It's a special case for how only!

Please ignore a couple of those comments: neither is correct. If you are keeping the same (general) word order, you would say "I know how difficult it is to learn a new language". This is the most common way you would phrase this sentence. You could also say, "I know how difficult learning a language is". But, I would go with the first.

the first is correct, but for the second one to be correct, it should be: "I know how difficult it is learning a new language."

Both are corrects