I have a fresh installation of Debian 9 (stretch) on my machine and I've set locales through
dpkg-reconfigure to a non-english language. Since I'm using KDE, I've also set this as the only language in the Preferred languages list in System Settings and I've installed the relative
kde-l10n package through
Now KDE-related programs are in the correct language, but all the others (Firefox, Thunderbird, Kaffeine, Telgram for example) keep displaying the relative interface texts in english. In particular in Firefox I've also noticed that the displayed dates use the UTC timezone instead of CET (which would be the correct one for my locale), and the French dictionary, which has nothing to do with my locale, was preinstalled for some reason.
env | grep LANG shows that the
LANGUAGE env variables are correctly set to the desired language, while
env | grep LC returns nothing (from what I've read
LANG is just a fallback variable in case no
LC_*s are found)
Why do programs not respect my locale and how can this be fixed?