New documents will behave, although you will need to fix old documents one-by-one. Then anytime you use that style in the future, it will automatically have the language tags. You need to modify the style that the text is formatted in, to include the language formatting. Fixing All Documents, or Changing the Default Language Spelling depends on the language setting, so first make sure you have a Tools>Language menu entry. Note: the Word 2004 Help topic 'Troubleshoot spelling and grammar checking' also addresses some questions not covered here.
Change Language in a Single Click in Word 2008 (external link, scroll down for English).Spell-Check Doesn’t Catch Misspelled Words.Updated for Word 2008 (DRAFT in Progress) Spell-Checking, Language, and Dictionary Issues in MacWord
