This looks quite nice and powerful and indeed an elegant way of solving the problem with multiple plural forms in a string. The only concern I have with that syntax is that it's yet another DSL, or markup language and translators need to know it, or could get it wrong. Granted, a program for helping translators might do automatic linting (much as Qt's Linguist already warns if you omit placeholders from the translated phrase that are there in the original).
Another thing is that the mini-language grows complex enough that the resulting text can be quite hard to read and understand:
{people, plural, offset:1 =0{No one went.} =1{{user1} went.} =2{{user1} and {user2} went}.} other{{user1} and # others went}}.