The Bilingual Confusion
Google Docs features an intelligent spellcheck and grammar system that attempts to adapt to your writing. If you begin typing a document using words from a language other than your default account setting (for example, you are writing an essay in French, but your Google account is set to English), Docs will sometimes detect this and automatically change the document’s language setting. This alters the dictionary being used, which can suddenly cause perfectly spelled English words later in the document to be flagged with red squiggly lines as “errors” because they don’t exist in the newly applied French dictionary.
Locking the Document Language
To maintain a consistent dictionary and stop Google Docs from automatically changing the language based on what it thinks you are typing, you must explicitly force a static language setting for the entire file.
While you cannot disable the auto-detect algorithm globally across your entire Google account, you can override it on a per-document basis. Open the specific Google Docs file that is exhibiting the issue. Press Ctrl+A (or Command+A on Mac) to highlight all existing text in the document. Navigate to the top menu bar and click on File. Scroll down the drop-down menu and hover your cursor over Language. A massive secondary menu will slide out listing dozens of languages. Scroll through this list and explicitly click on your desired primary language (e.g., English (United States)). This forces Docs to apply that specific dictionary to the selected text and locks it in for new text typed at the end of the document, overriding the automatic detection.