What does the Lang code "in" represent?



Based on https://dev.twitter.com/docs/api/1.1/get/help/languages, I implemented a class listing all the possible languages from the ISO 639-1 code.
I also listed all the languages from https://api.twitter.com/1.1/help/languages.json and I cannot get the lang=“in”.

Yet today after publishing a reply I received a json containing this value.
Could someone let me know what is this value supposed to represent.

Tweetinvi API team