Changing an admininstrator user language is pretty straight-forward, but it does not change the language of the Boot screen, Welcome screen, Login screen, etc...
Pretty much anything that's not on the desktop remains in the original language of the installation so it can look really ugly (for example, having a menu label in Spanish, and the menu items in English, etc.)
How to change that?
I've Googled this for hours and could only find answers about how to change the user language, which is completely irrelevant for this issue.