We're currently going through our internationalisation test phase for a major release and it's been a huge headache.

My one piece of generalised advice: DO NOT leave internationalisation testing to the end of the release. Make sure you're pumping Unicode strings through your app in unit tests. Make sure you're regularly viewing your app in other languages using mock strings if you haven't got the translations. The earlier you can find these defects, the easier it is to fix. There's a perception that internationalisation defects are easy fixes, just updating a messages file. The truth is that major parts of your app may not work unless you regularly test it. And don't rely on the Unicode abilities of your chosen framework, language or operating system. If you haven't tested it, it doesn't work