Another note is: if you experience locale confusion after logging in over SSH even though all’s fine when logged in locally it can be an e.g. stray LANGUAGE setting in the logged-into system’s /etc/locale.conf.
Not sure what sets that – may have been the installer – and also not sure what applies it but presumably pam or logind. In any case it’s likely the issue if a local session has no LANGUAGE set in its environment but an SSH one does. Simply deleting LANGUAGE from /etc/locale.conf manually is working for me.
when a secondary language is used for LC_specific settings like numeric or phone number.
The only case I have seen the LANGUAGE= is when you use Plasma systemsettings to change locale - in some complicated cases this creates a ridiculous confusion because a ~/.config/plasma-localerc is also read.