Offline Dictation feature (possibly) for 64-bit devices (only).
CydiaSubstrate part is already done, compile and try by yourself to see if this even works.
Alternatively, you may copy /User/Library/Assistant/SpeechProfile_LANGUAGE_CODE from iPhone 6s(+)/SE or iPad Pro and test on your own unsupported devices.
Supported iOS versions: apparently iOS 9.0+