Where there's a difference, we prefer American English spelling to Commonwealth 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 spelling style for consistency, and the founders of Wikitravel are Americans, so they just started writing American-style first.
If you are from a Commonwealth country and use "harbour" or "aluminium" or whatever, that's fine. We want your contributions and your work, so don't worry so much about spelling. But other contributors may come through and switch those over to American English, just for consistency.
Sometimes proper names have words in them that are spelled differently in American and Commonwealth English, or just misspelled altogether. These words should not be changed -- the proper name should not be mangled to conform to Wikitravel's spelling rules.
However, referring to the proper named thing in the generic should be mangled. Examples may be clearer:
See also: List of common misspellings