Using the PowerApp on web version is possible to "force" a language using the parameter "?l=en-us". But using the PowerApp studio Desktop or the PowerApp mobile, the application generated just can use the Operation System regional settings. In some countries people use to use foreign Regional settings and during the app creation we have to deal with it (users using different locales/language settings).
But, in some cases, we have to consider that every user have to be the same interface behavior, without changes and simplifying the app development/configuration. For this reason, I suggest the the value returned for the function "language()" could be updated in an App configuration or for another new function (setLanguage("en-US" for example)).