According to our most recent experiment, the answer is: 67 .
These are the top languages
This experiment was based on a random sample of 22.632.977 Tweets collected using Twitter Streaming API, 23.4.-7.5.2015.
We used the language attribute (“lang”) which comes as a data field of a Tweet to determine the language.
This experiment is still ongoing, the official results will be published in a research paper.