How to work with the "Tiếng Việt Telex" keyboard?

I have an iOS application that derives on UITextInput to enter text. I have also overridden pressesBegan() and pressesEnded() in order to have some extra keyboard management (auto-repeat, special actions for arrow keys, function keys...)

That works well for single-character languages (most roman languages, such as English, French, etc).

For multi-character languages (Chinese, Japanses, Korean, Hindi), I can detect that the keyboard has been set to that language, and switch back to the default version of pressesBegan():

if let keyboardLanguage = self.textInputMode?.primaryLanguage {
    if (keyboardLanguage.hasPrefix("hi") || keyboardLanguage.hasPrefix("zh") || keyboardLanguage.hasPrefix("ja") || keyboardLanguage.hasPrefix("ko")) {
        super.pressesBegan(presses, with: event)
    }
}

But that strategy fails with the Tiếng Việt Telex keyboard (for Vietnamese language input). The way that keyboard works (as you can see if you open a document in Pages) is that you type as you go: T-i-e-n-g V-i-e-t T-e-l-e-x and the system adds the relevant diacritics once you've finished a word, so typing "Tieng Viet Telex" gives you "Tiếng Việt Telex" on the screen.

Is there any documentation on the inner workings of this specific keyboard? What should I do (or not do) in order to make my application compatible with Tiếng Việt Telex?