A family of Microsoft word processing software products for creating web, email, and print documents.
In Word, language is a text attribute, similar to bold or italic. Therefore, you may run into trouble with proofing even though the default language of new documents is properly set.
In an affected document, select all the text and (re-)apply the correct language using the Language dialog box (Review tab > Language group > Language > Set Proofing Language). Confirm with OK (specifically do not click Set as Default). I would also recommend that you modify the language of the Normal style, if necessary.
For more, see Suzanne S. Barnhill's excellent article at http://wordfaqs.ssbarnhill.com/MasterSpellCheck.htm .
Fixing language settings will usually fix Word's AutoCorrect feature as well, because entries are language-specific.