Change spelling check language for a Document in Microsoft Word 2010

In Word 2010:

1. Select the text you want to affect. You can use Ctrl + A to select all the text in the document.
2. Review tab > Language group > Language button > Set Proofing Language
3. In the Language dialog, select the language you want to use for spell checking.

In Word 2007, Review tab > Proofing group > Set Language will accomplish the same thing.

Instead of selecting all the text, you can change the language for a specific style. Most styles are based on Normal, so you usually only need to change the Normal style.

In the Home ribbon, right-click on the Normal style and click Modify. Then click Format -> Language, and select the language. A blog post with screenshots is also available here.

This should also affect headers and footers.

This works in Office 2007 and 2010. In earlier versions it is also possible, but the process is slightly different.