Which desktop?
If KDE, checkout this thread or this one.
Locale will update /etc/locale.gen.
When selected, notice the Add button. There is also a context menu (right-click) on each added language.
Regional Settings will update files in your home directory like $HOME/.config/plasma-localerc. When selected, if you hover over a language, notice symbols on the right that can be used to remove the language or make it the default.
I use this command often to list changed files:
find -newermt '1 minute ago`
If they are system files, it needs to be changed to something like:
find / \( -path /dev -o -path /proc -o -path /sys \) -a -prune -o -newermt '1 minute ago'