Currently the main language of the Internet is English. However English is not the language spoken by most people on the planet. According to Wikipedia it comes in third place behind Chinese (Mandarin) and Spanish.
This site is multilingual, it can switch between English and Spanish. There is no "marketing" reason for this, I don't think I will gain clients from being multilingual. It is an example and quite affordable with any competent Content Management System, in this case Drupal 7.