iOS 9.3 offline dictation not working
I think I solved this!!!!

I have an iPhone 6s+ and iPad Pro, neither of which allowed offline dictation.

For some reason, the language package which allows your iPhone 6s(+) or iPad Pro to do offline dictation appears to be downloaded alongside of any of Siri's English dialects.

Discovered this semi-accidentally. I had a weird-and-shouldn't-work epiphany this morning. I had an American English keyboard but prefer Siri's British voice. I wondered if it would only download if the keyboard preference and Siri dialect were the same. Changed the dialect to American English (which I had to download), restarted the phone, and viola, offline dictation was enabled.

Tried the same on my iPad, but it didn't work. 

But then I realized that Siri's American English was already loaded on my iPad, so I tried downloading the Australian dialect, and voila!, offline dictation was now enabled.

It appears that downloading any English dialect for Siri will also download the offline dictation package.

Good luck!