Please help me edit the Font in JavaScript!

I like the theme of king size v3 themeforest
but now I'm having trouble. Font does not match the themes of my language, the default is English, but I use Vietnamese game it did not support this language
so I mistakenly Font, missing letters in a sentence
I write: Chào Các B?n C?a Tôi. but it just does: Cho CC Bn Cua Ti.
language is not English letters such as: â ? ? ? ? ? ? ? ? ? ô ? ? .......
I can not know how to program Javascript
I know here you can help me.
you please help me add the letters in the Vietnamese language into the JS file so I can use it in Vietnamese.

I have 2 attachments hope you can help me fix it.
my english is poor your sympathy

Thank you very much!