The Red Squiggles of Confusion
Google Docs features a powerful, dynamic spell-checking engine. In an attempt to be helpful, it tries to automatically detect the language you are typing in. However, if you are writing a document that frequently mixes languages—for example, an English essay that contains several long quotes in French, or a technical manual using lots of Latin scientific terms—Docs can get confused. It might automatically change the underlying document language setting to French mid-sentence. Suddenly, every subsequent English word you type is underlined with a red squiggly line because the spell checker is now evaluating your English against a French dictionary.
Locking the Primary Language
To prevent this erratic spell-checking behavior and stop Google Docs from automatically changing the document language based on context, you must explicitly force a static language setting.
Google Docs does not have a single “Disable Auto-Detect” button; you must override the behavior by enforcing a manual selection. Open your document. Press Ctrl+A (or Cmd+A on Mac) to highlight absolutely all the text in the file. Now, look at the very top menu bar and click on File. Scroll down the menu and hover over Language. A long list of supported languages will slide out. Click firmly on your primary language (e.g., English (United Kingdom)). By manually applying a language tag to the entire block of highlighted text, you override the dynamic detection engine. Google Docs will now strictly evaluate the entire document using only the dictionary you explicitly selected, regardless of what foreign words you type.