That's a personal opinion coming out of nowhere. Regional settings are set at installation time based on the choices of the user. Even when people move to a different country it's rare they update the regional settings because their native language remains the same, because they are used to their keyboard layout (not even mentioning that you can't change the keyboard on a laptop, etc).
Even if you ask some windows noobs how to change the regional settings, most of them will probably not even know and won't care at all.
MATE is a fork of Gnome 2 that was created when its development was discontinued. Initially, they forked the gnome-system-tools, but they discontinued it years ago (last was version 1.8 and current version is 1.22), do you really think that MATE would remain popular if it was only usable by advanced users ?
Last but not least, you have to know that this UI tool was quite limited. For example, if you want to have your TIME locale as English UK but you want to have your MONETARY, MEASUREMENT, ... as another locale, you couldn't do it with the GUI. Changing the locale post install, it's a one time command to run in a terminal and then you forget about it forever!
And if the aim is just to change the language, as mentionned on this blog post, we're going to introduce a new tool for user management (which will allow to change the language per user), but it hasn't reached a suffiscient manturity level yet:
Due to various issues with broken user management, we have decided to temporarily remove the packages responsible for providing this functionality, which are gnome-system-tools , system-tools-backend, and liboobs. We are actively working with the developer of an alternative tool to resolve various issues (such as avatar and language setting) and we are optimistic we’ll have a solution to deliver to our users. When this tooling is considered ready for production, we will deploy it automatically to our MATE users.
