'okina not available on iOS keyboard
It is great that Hawaiian is available now on the mobile version of Duolingo.
I have encountered one problem, however. Typing Hau'oli using an apostrophe, or the available variants on the iOS keyboard, was not accepted. There was no way to answer the translation of happy, whatever was typed was rejected and the question repeated. Eventually I quit out of the app. Possibly the correct symbol can be found somewhere on the iOS keyboard, but I couldn't locate it.
It would be good if using an apostrophe for the 'okina was just treated as a typo, as it is in the internet version of Duolingo, as this would avoid the problem of endless looping.
There is a Hawaiian keyboard on iPhone. Go to Settings > General > Keyboard > Keyboards > Add Keyboard, and scroll down to Hawaiian. It has dedicated 'okina and kahakō keys.