Question: I'm trying to build an app using oracle builder (oracle forms) and i want to know the user name after login, is there any option like sessions in (PHP/ JEE) to know who logged in my app?

Answer: 
I don't believe you can choose a fallback locale for a specific locale that isn't found. 

Instead you create a default locale that will always be used for fallback in the values/string.xml. 

If the device is set to a locale that your app doesn't have (e.g. values-fr), it will then just use the default values string.xml.
If there are untranslated resources, they'll fall back to the default file (the file that isn't set with a specific locale). You cannot change that behavior.

However, you can write a library for it, but since it is a library all references to `Context#getString` would call the native system which calls the default file and not a specific locale for fallback resources.