I think it's the locale files, like this one: https://github.com/timvisee/send/blob/master/public/locales/...
If you look at github/linguist, that's what recognizes languages in repos. It has this rule for FreeMarker: https://github.com/github/linguist/blob/32ec19c013a7f81ffaee...
It seems a .ftl extension means FreeMarker to linguist, so those localizations show up as such.