The Confused Dictionary
Google Docs features a powerful, cloud-based spell checker. By default, Docs attempts to automatically detect the language you are typing in. If you are writing a document primarily in English, but you insert a long quote in French, or type several Spanish names, the Google Docs algorithm might suddenly decide that the entire document is now in a different language. It will automatically switch the underlying dictionary, instantly painting your English paragraphs with thousands of red jagged underlines as it attempts to spell-check English words against a French dictionary, creating massive visual clutter and rendering the spell checker useless.
Forcing the Default Language
To ensure consistent spell checking and stop Google Docs from automatically changing the language dictionary based on what you type, you must explicitly define the language for the entire document and disable the auto-detection.
First, you must force the document back to your intended language. Press Ctrl+A (or Cmd+A on Mac) to highlight all the text in your entire document. Look at the primary menu bar across the top of the screen and click on File. Scroll down the dropdown menu, hover over Language, and explicitly click on your desired language (e.g., English (United Kingdom)). This forces the current text to use that dictionary. However, to stop it from switching back automatically, you must adjust your global account preferences. Go to your main Google Account settings (myaccount.google.com). Click on Personal info on the left. Scroll down to “General preferences for the web” and click on Language. Ensure your primary language is set correctly, and if there is a toggle for “Automatically add languages” based on your activity, turn it Off.