It is said as so because of the way in which it can be used. Other languages tend to have, for one, not nearly as many words. The dictionaries of other languages, for years and years, did not need to be updated every year until English came around, and started taking words, roots, etc, from every other language to make new words for new things, make new words for old things, and just generally make new words.
Also, compared to most of the languages in the world, English is backwards. It's why someone (my Grandfather) whose native language is Greek can learn Spanish damn near perfectly in no time at all (about 2 years), and yet, to this day, he still has some trouble with English (He's lived in the U.S. for over 40 years).