The Confused Dictionary
Google Docs is built with intelligent language detection. As you type, the software analyzes the vocabulary and automatically switches its spellcheck dictionary to match what it believes is the primary language. While this is incredible for bilingual users who frequently switch between typing paragraphs in English and Spanish, it is a nightmare for users writing highly technical documents. If you are typing an English document heavily peppered with Latin legal terms, French culinary phrases, or specialized programming syntax, Google Docs will often mistakenly assume you have changed languages, instantly red-lining the rest of your perfectly valid English text.
Forcing a Single Language
To maintain a consistent editing environment and stop Google Docs from automatically swapping its spellcheck dictionary based on perceived vocabulary changes, you must disable the auto-detect feature.
Open any Google Document. Navigate to the top menu bar and click on File. Scroll down the dropdown menu and hover over Language. A secondary menu will expand to the right. Scroll to the very top of this extensive language list. Here, you will find an option labelled Automatically detect language, which is usually checked by default. Click on it to uncheck the option. Once disabled, you must return to the File > Language menu and manually select your primary language (e.g., English (United Kingdom)). Google Docs will now rigidly enforce this specific dictionary for the entire document, regardless of how many foreign loanwords or technical terms you incorporate.