Can I change a document's default language?

"I want to change the language of the document I'm writing. I just need to change it for an individual document, not the whole platform. Can I do that?" 

Of course, you can change the language of your document.

In your workspace, click on the gear wheel icon in the top right and select "More settings".

Here you can select the language you're writing the document in and click "Submit" to save your changes.

If you want to know how to change the language of the entire platform, read this other article.