Occasionally, I'll have need to translate text from one language to another for testing purposes. I don't do a lot of internationalization testing, but many sites today have simple text referencing an alternative language (like Spanish in the United States). For simple text conversion, I've found Google Language Tools to be quite useful. It's not going to let you take up that new project in a language you don't speak, but it might get you by in a pinch.
When testing sites (or even applications) that support multiple languages, often the static text is "professionally" translated. What I really mean by "professionally" is that it's not dynamically translated. Someone sat down and wrote it out. That's not neccessarially the case with dynamic error messages. (Or really any dynamic aspect to the applicaiton.) When you're testing different languages, be sure you're exercising the aspects of the application that are dynamic (like error messages) and see how they read. It doesn't always have to read perfect in multiple languages, but it shouldn't be awkward.
When testing websites for internationalization (language switches, etc...) look for transactions that transition you from one website to another. It doesn't matter if the transition is between websites internal to your company (like a marketing website) or external (like paypal). Many times you'll find that when you're returned from the other site, you're returned to the default language for the website you're testing. Not the language you were in when you left.