Thanks. Niklas, this notation is not one we have seen before. Should we consider advising jquery.i18n? https://github.com/wikimedia/jquery.i18n
The notation is new to me as well. Will likely confuse some translators too.
It's documented on the project page: https://translatewiki.net/wiki/Translating:Hive.js#Plural
I didn't invent it, but the general idea of this syntax is to use curly braces to take you into "meta mode", use them again to go into "literal mode" again.