I believe the original intent of HTTP/2 (all HTTP/2) was to use ALPN to allow for faster encrypted connections, due to the application protocol being determined during the initial connection.
To enable ALPN on Nginx, I believe you need to be using OpenSSL 1.0.2 on your server.
You can determine the version of OpenSSL you have installed on your server by running:
...from the command line.
Additionally, you can actually check to see if ALPN is installed/used by running:
echo | openssl s_client -alpn h2 -connect example.com:443 | grep ALPN
...or by navigating to https://tools.keycdn.com/http2-test in a web browser, typing in your URL and clicking the test button.