How to change language of a single Mac app on Sierra?
One way to do this is to go into the app (control click, show package contents, contents, resources) and switch the names of the .lproj files, so that the one corresponding to your OS language actually contains the data for the language you want the app to display in.

This will only work if the application in question is properly localized for the locale you choose. Also, be sure you have a backup copy or plan to reinstall the app in case modifying it breaks anything.