Is your feature request related to a problem? Please describe.
It's a common requirement to turn a quantity of something in a message format into something that is grammatically correct. You can have "1 message" or "2 messages" in an English example. For a language like English and many European languages, it's not that hard. If you ever need to pronounce the number, then it's a little harder because you would need to use ICU's rule based number format (RBNF) to do that. If you need to refer to user vocabulary, it's really hard to do normally, especially when you need to apply grammatical case to both the number and noun.

Describe the solution you'd like
I would like some syntax that takes a number and a noun phrase and get them into grammatical agreement. It would use CLDR plural rules to determine the grammatical number form to use for the noun phrase. Ideally it can provide the spoken form of the number by using rule based number format (RBNF) from CLDR. Then it can take the grammatical case of the entire phrase to get them both into agreement.

Describe why your solution should shape the standard
This is really hard to do in message formatting right now, and if it is doable, it requires a lot of copy and paste syntax to do the same thing in Arabic, Hebrew and Russian. It should be as simple as English.

Additional context or examples
Siri already does this in its responses.
Let's Come To An Agreement About Our Words
The following are prerequisite issues: #33 #31 #16