Thanks, I was referring to john doe's earlier comment "In other words, one language needs to be selected in order to be able to choose 'none' (use none if you access the host through SSH) or 'C.UTF-8."
And the fact that in dpkg-reconfigure locales, I didn't see the option for C.UTF-8. If I uncheck all other locales, the ascii gui doesn't even give me a choice to select the default locale ( I was expecting a choice between none and C.UTF-8). In order to let the gui give me a choice in the end, I have to choose at least one locale ( I think this is what john means as well ).