Apple's new iApp Pages makes beautiful documents. I like the feature set a lot, too. But there's one bewildering setting that took me forever to figure out: multiple language support. If you have your OS set in one language (like English), and want to write in another language (like French or Spanish), Pages refuses to change the language of the file to anything other than the language setting of the OS itself. So you get the red squiggly lines everywhere. I did find a way to set the language for the currently selected text, but no way to apply the change document-wide without doing a Select All, and no way to set the language to be another one permanently.
To set the language for the currently selected text, bring up the Formatting palette. Under Text > More: Language, you can set the language. To set the language document-wide, first do a Select All, and then change the language. It took me a while to figure out that Pages can have multiple language types in a single document. This is why when you change it in one place, clicking into a different block of text reverts back to the default language.
I filled a bug report with Apple, but apparently this is the "intended functionality". Not very intuitive, but it makes sense if you want to write in multiple languages in the same document.
Mac OS X Hints