While "a lot" of the users may speak English as a second language, a website needs to use a common language for all users in order to be useable. If parts of the site were in Tagalog, Tamil, Urdu, Spanish, Finnish, Cantonese, or Basque, then people who don't speak those languages would not be able to use parts of the site.
Since the site is operated by Americans and targeted to readers in the US, Canada, UK, Australia, Ireland, New Zealand, South Africa, and other English speaking nations, it makes sense that the common language be English. English is commonly spoken as a second language throughout the world, so having the site in English helps Hubpages content reach a wider audience.
Translating the site's content into various languages would require Hubpages to buy more servers and significantly slow down the site. Why would they do that when readers can use free online translation tools to render hubs in their own languages? Google Translate isn't perfect, but it is free and easy to access.