How to change link of buttons

Hello,

I don't want to use defauls link of language

*** Option 1:
I have two language that it's Vietnam and English
In header of web that I want change link flag English. When click to flag English that it will link to http://hbc.vn/en/

*** Option 2:
I have 1 language, it is English
In the header of web that i want link to http://hbc.vn/vi

Help me plz

Thks u so much!

Who can help me ?