These suggestions don't help for people (translators, teachers, etc.) who have documents with two or more languages, or two language varieties like the original post. You used to be able to select a portion of the text and set its language to avoid incorrect autocorrecting and/or red underlines. This was separate from the system language settings.
Word can do this, but it's a pain. It used to be easy and effective in the previous Pages for the Mac. This missing feature is a deal-breaker for me, and I would think for the OP as well. I won't be able to use the new iLife suite unless there's a way to set different languages within a document.
I've looked at a few threads here and elsewhere and don't see any solutions proposed for this yet, so if anyone knows if this is possible it would be helpful. If it isn't possible, people should submit feedback to Apple to ask them to bring it back.