A family of Microsoft word processing software products for creating web, email, and print documents.
Could it be that your normal template has been switched to default to spanish. Create a new blank doc and check the proofing language it defaulted to.
If the switch seems to happen "on the fly", it could be that you have a shortcut set on the windows keyboard languages.
Take a look at Control Panel > Region and Language > Keyboards and Languages tab > Change Keyboards... button >Advanced Key Settings. I assume you will find more than one keyboard installed, check the shortcuts.
Maybe you are hitting the shortcuts by accident ... switching language in the middle of the document...