Commit c947b4f removed references of any country code in the Persian translation but did not rename the file containing the compiled translations accordingly. In other words, fa.js does not exist because fa_IR.js was never regenerated from the updated translation file.
https://github.com/m-lab/mlab-speedtest/blob/4cb40a00cc59ad46f8c7c1912461614c2bdc40a7/app/fa/index.html#L65
Commit c947b4f removed references of any country code in the Persian translation but did not rename the file containing the compiled translations accordingly. In other words, fa.js does not exist because fa_IR.js was never regenerated from the updated translation file.