Not official statistics by any means so this is about as close as we are going to get. 2017-2018 tracker and 2016-2017 tracker
I think the acceptance rate is a result of just how niche the fellowship is. I can't imagine a lot of grad students want to take a year off to live in a foreign country (a not so cushy one at that) and learn a (objectively difficult) language and then work for the government. But it's nice, because if all of that makes sense for you, then you stand a good chance!