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.
New Zealand is a particularly mono-lingual country. Our education system (apart from Te Reo) offers languages other than English quite late (usually secondary school) and the languages provided are not useful in terms of our major trading partners. Few websites in the tourisn sector are multilingual, congratulations to those who have made the effort. I believe there is a good case for tourism operators to consider selling themselves to people whose first langauge is not English. Site visitors would be impressed and more likely to take up the products/services offered.
Assuming the entire world understands English is not true and probably impolite.
Please contact us if you would like more information about making your site multilingual.