How to get Pages in Mac Yosemite 10.10.05 to recognise different languages for spelling?
In Pages 5 or 6, go to Edit > Spelling and Grammar > Show Spelling and Grammar and set the dictionary to the language you want (not Automatic).

In Pages 4, go to Inspector > Text > More > Language and do the same thing.

If you want to try Automatic, which may or may not work very well, be sure to go to system preferences/keyboard/text/spelling/setup at the bottom of the list and check only the boxes for the languages you want.

(The OS version doesn't matter)