I’m currently working on a site that needs to be truly localized for a large number of languages (tens of them). This is accomplished with large numbers of ResourceBundles, the MessageFormat class when variable text layout is needed, an Oracle backend which understands and doesn’t muck with UTF-8, an Access database which generates said bundles, and a crack team of translators.
However, how to test? Luckily, it’s fairly easy to have IE use a different language: clear instructions live here. One issue with the instructions is that they don’t tell you how to actually install a language pack. But this is easy too: I only had to right click on a page, choose the encoding menu, then choose more, and then the encoding I wanted (Korean, because I want to test double byte characters). I was then prompted to install a language pack. I accepted, and Windows downloaded a bunch of DLLs and other files. From then on I could view Korean characters (the encoding menu says I’m viewing ‘Unicode (UTF-8)’). Here’s a random site about mining that you can use to test your Korean language pack.
Don’t forget to test both the input and output of your application–saving user input, and being able to redisplay it is at least as important as being able to display what you draw from your ResourceBundle initially. As a bonus, the Korean character set that I installed via IE was made available to Firefox. This was done on the fly, not only did I not need to restart Windows, I didn’t even need to restart Firefox; I just needed to reload the page.