Does ProWritingAid use British or American English?

ProWritingAid has options for British, American, Australian, and Canadian English. You can choose which version you want by changing your account settings .

In the online editor the settings are found at the top right of the screen: 

In the desktop app you can find them in the tools menu: 

The language in the Office add-in for Word and Outlook is determined by the proofing language already selected. To change it go to Review > Language > Set Proofing Language