Make application automatically detect system language

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http://askubuntu.com – What should an application developed under a Linux System like Ubuntu do so as to automatically detect the system language? There are applications, like Liferea that automatically change their language to match the system's, without altering any preference of the program itself: Should this be the "default" behavior for all the programs? Should there be an option on the program so as to let the user choose the language nonetheless? Are all these translations coming along with the program itself? What if the user has set a system language not available in the translations of the program? Is t (HowTos)