Override language setting for some apps?
Somebody mentioned Tasker? So here I am. And in fact you could do that with Tasker and the help of the Secure Settings Plugin:


Create a new task:

Plugins -> Secure Settings -> Locale Picker (pick your target locale here)
App -> Run (select the app to run)
Plugins -> Secure Settings -> Locale Picker (pick your original locale here)

Assign an icon to this task
On a free place of your homescreen, long press, add a tasker widget, pick this task


I did not test it, but it should work like this.