Translations:Extension:UniversalLanguageSelector/19/en

First of all, you need to find or produce such a free font (this is the most important part and you have to do it yourself); then, it has to be converted to the required formats, which are eot, ttf, woff; finally, you can [$phabricator-req file a request] in Phabricator for the font to be added to the extension.