Well, actually it’s a problem in the English locale file that’s causing this.
server.en.yml contains the zero, one and other keys, but according to the official pluralization rules only one and other are allowed. It currently works for English (and all the other languages that use only one and other) because the Ruby I18n module has a special case for the zero key.
https://github.com/discourse/discourse/blob/master/config/locales/server.en.yml#L156-L159
Unfortunately Transifex doesn’t handle the special case of zero, so it doesn’t detect any key as pluralized if there exists anything else than one and other in the English locale files.
That’s the reason why the server.cs.yml contains the zero, one and other keys instead of the required one, few and other.
https://github.com/discourse/discourse/blob/master/config/locales/server.cs.yml#L110-L113
There’s currently no easy way to fix this. I was planning on proposing a few solutions to this problem in the next few days.