OS X has had a system-wide, built-in spell checker for a while now, but until Snow Leopard, it could only check the spelling of whatever your default language was. But what if you needed to prepare a document in another language, say for a college Spanish assignment? In that case, you'd end up with a document with pretty much every single word underlined in red, with no reliable way to spell check it.
But now, OS X offers simultaneous spell checking not only in four different varieties of English, but also in Spanish, French, German, Italian, and six other European languages. You can mix and match these languages in a single document, and the built-in spell checker will intelligently adapt to whichever language it thinks you've switched to. Pages from iWork '07 doesn't seem to benefit from this new feature, nor does the 2008 version of Word, but it works just fine in Safari and TextEdit. With TextEdit you get an added feature: once it figures out what language you're typing in, autocorrect will work for that language just as well as it does for English.
So, for example, when you write in Spanish, the computer's dictionary knows it has to look for words in Spanish.
Or, if you'll forgive mi español descompuesto,
Entonces, por ejemplo, cuando tú escribes en español, el diccionario de la computadora sabe que tiene mirar por palabras en español.
That last sentence would normally have red underlines under nearly every word, but using TextEdit in Snow Leopard the spell checker adapted to Spanish spelling as soon as I finished typing "entonces." It also auto-corrected espanol to español, which is much easier than having to type option-n, n to get the tilde above the n.
The adaptation seems to happen on a paragraph-by-paragraph basis. In other words, the spell checker doesn't seem to be intelligent enough to recognize when you switch languages in mid-paragraph, much less mid-sentence. The spell checker will do its best to figure out the primary language of the paragraph; for example, if you type a few words in English but the rest of the paragraph is in Spanish, the English words will show up as misspelled.
There's some potential for confusion if you switch back and forth between languages within paragraphs, but between paragraphs there's no apparent issues.
This would have come in really handy a couple of years ago; after opening some old Spanish assignments I had, I found some of my compositions riddled with minor errors (mostly misplaced accent marks) that the spell checker in Leopard or Tiger never would have caught.
¡Viva la Mac!
View the video below for a brief glimpse of the new spell check behavior in action: