Wikitravel recommends using the local variant of English for all destination articles. This means:
Don't worry if you're not familiar or comfortable with the local variant of a destination: just write in the style you're accustomed to. In any rate, the only case in which it would make sense to go through an article to standardis(z)e the orthography would be in preparation for a star nomination.
If the destination has no history of using English and no clear preference for the variety to use, we prefer American English spelling. This isn't because American English is somehow better or to stomp on the rights, heritage, and cultures of other English-speaking countries. We just need to pick one default spelling style for consistency. Below are reference guides for American spelling:
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