A family of Microsoft word processing software products for creating web, email, and print documents.
Spellchecking in Word depends on the language format applied to text in documents. "Detect language automatically" often causes problems, because it instructs Word to guess the language as you type, and to change the language at the insertion point according to those guesses—which are often incorrect.
If your document is written is one language, select all the text with Ctrl+A and re-apply the correct language via the Language dialog box (Review tab > Language group > Language > Set Proofing Language). Also clear "Detect language automatically" as well as "Do not check spelling or grammar" in that same dialog box.
For more, read Suzanne S. Barnhill's excellent article on the topic: http://wordfaqs.ssbarnhill.com/MasterSpellCheck.htm .