Without doubt, English is the most common language for tweets. However, thanks to Twitter’s slowdown in the US and it picking up fast in many other parts of the world, the share of English tweets has come down from 67% to 50% in just over six months.
Here are the top five languages on Twitter and the share of tweets they represent
English : 50%
Japanese : 14%
Portuguese : 9%
Malay : 6%
Spanish : 4%
For a perspective, compare this with this list of the countries with largest representation on Twitter