Users sometimes complain in the Apple Discussions about the problems that can arise in coordinating the selection of keyboard layouts, fonts, and spell checking when doing multilingual work in OS X. Anyone in this situation may wish to have a look at Nisus Writer Express or Pro (trial downloads available).
The Nisus apps have preference settings where you can determine the keyboard layout, font, and spell checker for any "language" you select from the list produced by clicking on the toolbar Language icon or the Format > Language menu. Plus you can define custom "languages" and also set a keyboard shortcut to activate any individual language on your list.
I haven't myself come across any other word processor with such flexibility in this area. If readers know of one, post a comment.