I want to learn english!

-------learn is better because in this case it is something you would like to accomplish.

-learning means that it is something you are doing right now, for example "right now I am typin" meaning, I am doing it right now. --I hope this helps :)------

I have some account on language learning site. so people who are learning (or want to learn) Korean, Japanese, or English can add me as their friend.

---I was not sure if you were talking about having other accounts on different sites ....---- if you are exppressing that you want friends n this site you can say "I would like to make friends on this site" or "You can add me as a friend ..."-----

 That's all! Please let me know where I am wrong. Tell me what is the better expression! Or, How bad my English is.

*Keep practicing and you will be better at Enlish very soon. Good job on writing this post*

:)

다이아몬드 

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