If you want to add your language to the game, clone this repository, copy the en directory and name the new directory according the Language code reference (lookup app), translate the files inside the new directory and issue a pull request. You may also send the files directly, if you don't want to use git.
Copy and translate the untranslated files from en or modify the current files in the directory of the language you want to modify.
game.php - every language must have this file, covers most of the game interface
frontend.php - (optional) strings used only by the javascript game fronted
web.php - (optional) all other strings used on the website (user profiles, leaderboards, statistics, replays, etc.)