I need to add a note here:

The Google keyboard (GBoard) for Android supports Navajo, but it seems to show the same problem with the apostrophe, so I think it will show a wrong answer.

Yáʼátʼééh! This Navajo course is using a special mark called modifier letter apostrophe, very similar to the right single quotation mark (it is a different curly apostrophe, instead of the vertical apostrophe), so words as yáʼátʼééh are showed incorrect when they are written as yá’át’éét (closing quotation mark; there is a little difference with this apostrophe) or yá'át'ééh. Please, try copying the three examples in bold and paste them on the notepad, then select all and change the font to Verdana to see the differences; other fonts will show the Navajo glottal stop and the curly apostrophe as the same. The apostrophe used by Duolingo will be showed as a square dot with a curly tail.

Here are the instructions to install a Diné Bizaad keybord layout:

Installing and Using the Navajo Keyboard - NavajoNow.com

https://www.youtube.com/watch?v=TIaShnHXVmQ

On Android, Multiling Keyboard can be used for Navajo.

I hope it helps!