The language is a global setting mostly because of all the integrations we use how would we ever know what languages the forum use we only support specific ones.
I can look at adding in 8.5 the ability for the user to set the language via a dropdown menu maybe in the menubar and set a cookie. If that cookie is present in the browser that would allow choice of the supported languages our application has. I could even add a profile option to set the default language a user wants as a profile option.
You can not install multiple copies of PhotoPost on one domain as that clearly is a violation of your license agreement.
Just for your clarification as I know you have seen it before since it is right on the order page