The iPad became available today. As noted in an earlier article, the language capabilities (listed in the tech specs) are rather less than those on the iPhone/iPod Touch. In particular there is no input for Greek, Arabic, Hebrew, Thai, Korean, Traditional Chinese, and also various European languages (see list here.)
iPad Safari and Pages can display all of the missing scripts without problem, however. As in the iPhone/iPod, support for Indic scripts, such as Devanagari, Tamil, and Tibetan is still not available. There is no ability to add additional fonts. A list of included fonts can be found here.
International Keyboards are discussed on pages 19-20 of the iPad User Guide. This states that "some languages written from right to left" are supported, which is clearly wrong at this point.
Japanese Input includes qwerty, azerty, and qwertz Keyboard Layouts, but no direct Kana input. Chinese, in addition to handwriting, provides qwerty and azerty.
There is a dual set of keyboard layouts in Settings, one for the virtual keyboard and one for hardware keyboards. For example, under English Hardware, you can choose US Extended, US International PC, and Dvorak, among others. A full list of layouts is here.
To use a JIS (Japanese) hardware keyboard, set the iPad OS to Japanese before trying to pair it.
As of 7/28/2010, the iPad app store lists keyboards for Arabic, Persian, Hebrew, Thai, Vietnamese, Greek, Korean, Dvorak, Norwegian, Swedish, Turkish, Czech, Ukrainian, and Macedonian.
Apps that let you input all unicode characters include Unicode Maps, Unicode, and Unicode Table.
Apple says iBooks can display books written in English, French, German, Japanese, Dutch, Italian, Russian, Simplified Chinese, and Spanish. The built-in dictionary is only available in the English language. I have reports that iBooks can also display other languages, but that the page order is wrong in RTL scripts.
VoiceOver speaks 13 languages and works with all of the applications built into iPad. The list of languages is that found on this page.
Pages 1.0 is only available in English. A 5/13 update adds French, German, Japanese, Dutch, Italian, Russian, Simplified Chinese, and Spanish. Pages cannot display advanced typography in the Hoefler Text font.