Hi duyhoa83,
Please note that in the current Little Reader version (2.0), you can't edit courses yet -- creating your own course or adding lessons to the existing English curriculum is not yet possible.
What you can do for now is to create a Playlist containing your translated lessons, which you will need to play separately from the English course.
I noticed that you are from Vietnam, so I'd like to direct you to the Vietnamese version of our "how to translate" article:
http://support.brillkids.com/entries/21313529-lam-sao-e-chuyen-bai-giang-tieng-anh-trong-little-reader-sang-ngon-ngu-ma-toi-muon-day(We are still in the process of translating all the support articles, so you'll notice that there are only a few Vietnamese articles at the moment)
Features for editing courses will be available in Little Reader v3, which is currently in the beta testing stage.