In order to make sure your document doesn’t contain any spelling issues, turn on the spell check (it’s available for 34 languages). Right now this is a global setting, if you need to use a different language in a specific document you can quickly change it in the document details.
Click on the avatar on top left, select Preferences, and click on the Language tab. Select the language from the Spelling dropdown list.
Click on the avatar on bottom right, select Preferences, then tap on Language and Spelling. Select the language from the Spelling dropdown list.
2. Document Details
If you want to change quickly the spell check language, you can do it directly from the document as well.
Under the 4th tab on the left sidebar below Location.