A family of Microsoft word processing software products for creating web, email, and print documents.
In Word, there is a difference between the default proofing language and the language which in use for a particular text range.
To prevent Word from trying to guess the language and change languages at the insertion point, clear the check mark next to "Detect language automatically" in the Language dialog box (Review > Language > Set Proofing Language). This modification applies to future edits in all Word documents.
However, for existing documents, you will have to select all the content (Ctrl+A) and manually (re-)apply the correct language, again via the Language dialog box. When you do this, do not choose the Set As Default button; only choose OK. See also MVP Suzanne S. Barnhill's article at http://wordfaqs.ssbarnhill.com/MasterSpellCheck.htm.