Can I change the spell check language to e.g. Spanish?
Yes, you can change the spell check language to your preference (e.g. Spanish) for a particular document via the project settings menu -- simply click the 'gear' icon in the top right of the editor to open the menu, and choose the spell check language from the dropdown:
You can also change your user settings so that your preferred language is the default spell check language for new documents you create.
For a full list of languages that are supported by the integrated spell checker on Overleaf, please see this FAQ answer.