Now On Tap is missing from Marshmallow
Apparently (as of current writing), Google Now on Tap only available if the System language is set to English.

From Official Google Support for Search,


  What languages is this feature available in?
  
  Now on tap is currently only available in English.


Perhaps an understatement, since setting the language to English in Google Search app doesn't show the option for Now on Tap.

According to Reddit,


  Now On Tap only works with English system language.
  
  Just a heads up.
  
  I tested it, and it checks out - with system language set to English, you'll find a Now On Tap toggle under Settings - Google - Search and Now. In another language, that toggle isn't there.


As for the follow-up, it seems any English language is accepted, not only English (US) or English (UK)


  Any English (UK, AUS, India)? Or just US English?
  I guess I could try it and find out but goggle wont gib oats!
  
  
  US and UK worked for me.
  Working fine on English (India)
  English AU works fine