Hi, i purchased theme from envanto. I am in Vietnam and want to change language from English into Vietnamese such as:
Hi,
This really depends on what type of theme you have purchased (HTML / Wordpress / Joomla etc.)
I think the best way would be to contact the author and ask if he/she can explain how to translate the theme to your language.
Hope this will help you out 
I am using wordpress theme, this is my site: http://saigonbuy.vn