Getting correct English grammar
It seems like you would need to write 3 different sentences for every out you want.
Format strings can help here (maybe not in the edge cases - however you have given no example of those)

"%s rock%s!".
The first string is then either "You", "He", or "She".
The second one is for the "He, She, It the s must fit" rule.

What about the am/are/is/... distinction? You could configure a place holder

#define ARE_IS "{AREIS}"
const std::string DUMB_MESSAGE = "%s" ARE_IS " dumb!";


and then replace ARE_IS with the correct form.

How fitting this solution is, how flexible and how much work you will have with that - we can't guess that with the information given.