Just so the solution is not lost.
"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."