Ok, so users can do this already if they know how to start the application from a CLI. Sometimes the command line is not obvious even if you know the application's name.

Maybe it is not needed to have a GUI command in every application then (in Help/ -menu for example), but as Juise- said a separate GUI program which would know all applications installed and could run anything with LC_MESSAGES="en_GB" enabled.

Those who are making bug reports and feature requests, can probably use also the CLI method, but those who are just searching for help from forums and from FAQs defenetly could use an easy GUI-mode of doing this and switch back and forth from en_GB and their native language.

Does any developer know could this be done on the fly when the program is running? (or is the language fixed in the initial mapping stage of the program run?)