Vietnamese - character automatically changing

Hello, I've been building a bunch of simple courses in different languages, all based on power point imports. 

Vietnamese is proving to be a real issue. The text will look fine in a text box, but as soon as I click on the text box, it decides to change characters. If I correct them, they change back when I click off, it's very frustrating. 

For example, this text

Nhấp Kế Itếp để Tiếp Tục

will change to this

Nhấp Kế Itếp để Itếp Tục

If it does it twice in a text field, I've had to make images and paste them in, which is looks quite average and is extremely time consuming, its not really a good fix.

I've tried changing fonts, but that has not helped, is this a known bug, or is there a known fix?