I did it at my base education center. They gave me an example of the 4187 I had to have sign by my Commander; then I made an appointment (or show up during the testing hours) and did it. Been on my records every since (obviously due to passing). A number code is given for read/write/speak for the language you passed on your ERB/ORB.
that is a link that basically relates to my above explanation. My language does not earn any extra language pay but if you know those Afghan dialects; it great to pass that test and get it on your records.