There is no input&language option in settings, and the settings.apk didnot work on stick 4k（the gen 3 stick）,so the only way to change the input method is using the adb link.
Install shouxin apk (or other input method you like)
adb link the stick
adb shell ime list -a
(you can check out all the input methods on the device. Confirm the name "com.xinshuru.inputmethod/.FTInputService")
adb shell ime enable com.xinshuru.inputmethod/.FTInputService
adb shell ime set com.xinshuru.inputmethod/.FTInputService
May be it's not useful for English-speaking susers. But for the asian users it is important to change the input method.
Enjoy the stick!
Something for Chinses Users:
I am new here, cannot thread on the "FIRE TV ANDROID DEVELOPMENT",maybe need ten .Would you please give me a ? Thanks!