If the destination has no history of using English and/or no clear preference for the variety to use, we prefer American English spelling. This isn't because American English is somehow "better" or because we want to stomp on the rights, heritage, and cultures of other English-speaking countries. We need to pick one default spelling style for consistency, and the founders of Wikitravel are Americans, so they just started writing American-style first. Below are reference guides for proper American spelling:
Don't worry if you're not familiar or comfortable with American spelling and want to edit a place in the "American sphere". Somebody else will come along and fix it up.
Occasionally proper names do not follow local spelling conventions. These words should not be changed -- the proper name should not be mangled to conform to Wikitravel's spelling rules.
See also: List of common misspellings