How to make a Linux system only use english as a locale and language, but use another one on a GUI/Desktop Environment?

Let's say that I install a Linux in my native language (english), and that the user I'm doing the install for only knows spanish (which all I know of is: Que?). English being obviously far easier for me (and most people) to use if I need to do some sysadmin when it's needed, so does the user's life when the GUI/Deskop Environment and GUI softwares are in spanish. The OS is Debian, which is installed from a netinstall in english so I can have the whole system in english, then install a GUI/Desktop Environment and keyboard that'll be in spanish. Now, I do know that most softwares and Desktop Environments do match their languages on the system's language, which is quite not good for what I intend to do. There's also an issue I have faced, which is when the system is installed in english, but that the keyboard is set to spanish from the Desktop Environment. That is, the keyboard is still in english despite having set XFCE's system tray flag (the plugin that allows one to change the keyboard) to spanish before shutting down, which still remains in spanish (the flag) after the reboot, and that spanish is obviously set as the preferred top language in XFCE's settings. So there must be a way so that the system stays in english (CLIs, terminals, etc), but the GUI/Desktop Environment (XFCE) and softwares remain spanish. How can I do that?