locale: Reset lost settings [duplicate]
Happens to me occasionally too. Not sure what causes it but I just fire off:
sudo dpkg-reconfigure locales
And that seems to fix it (for me)
In case you've deleted some files, try reinstalling the locale package:
apt-get install --reinstall locales
You may want to do the same for language-support-(your langcode), language-pack-(your langcode)-base and other language packs (gnome, kde...) for your language.