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
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