Once the game has been downloaded and installed, updates are running very slowly so be patient. After all of that is complete you'll be brought to the login screen, the language as you already know is foreign (Vietnamese). To make it a bit easy for those of us that don't understand any other language than english, here's some login translation to english to make your signing up process hopefully a bit more easy.
MU-ORIGIN Dang nhap
(Vietnamese language translated to english)
Nhap ki tu hoac email = Enter your email
Nhap M. khau = Import M. password
Dang nhap = Login
Dang ky = Registration
Choi thur = Play try
Tim mat khau = Find password
Doi tai khoan = For accounts
Vao game = Enter the game