Dictionary.com
Lots of references including translation dictionaries, style guides, a thesaurus, and word games. (Some services cost money.)
http://dictionary.reference.com/

Ethnologue: Languages of the world
An enlightening site about all the ways people communicate worldwide
http://www.ethnologue.com/country_index.asp

Consider Your International Audience
Overview by web analyst Larisa Thomason of issues to consider when writing
http://www.netmechanic.com/news/vol6/design_no15.htm

Are You Cultured? Global Web design and the dimensions of culture
Very thought-provoking article by Aaron Marcus of New Architect
http://www.newarchitectmag.com/documents/s=2450/na0303r/index.html