- User had half-broken locales in Arch Linux for 8 months, with errors like 'Cannot set LC_CTYPE to default locale'.
- After checking /etc/locale.conf and /etc/locale.gen, the issue persisted.
- Found clue: unsetting LANG and sourcing /etc/profile.d/locale.sh resolved the problem temporarily.
- Realized KDE was overriding locale settings with a misconfigured value (en_us.UTF-8 instead of en_US.UTF-8).
- In KDE Settings -> Region and Language, 'American English' was listed twice; deleted both and re-added.
- After running locale-gen and rebooting, locales worked correctly.
- Theory: initial typo in /etc/locale.conf (en_us vs en_US) was fixed later, but KDE stored the bugged entry.