As far as I can remember, this bug has been appearing regularly over the last fifteen years or so.
My system is installed in French. Firefox is installed, as well as the common translations package.
[microlinux@bernadette:~] $ echo $LANG fr_FR.UTF-8 [microlinux@bernadette:~] $ rpm -qa | grep -i firefox MozillaFirefox-translations-common-78.0.1-lp188.8.131.52.x86_64 MozillaFirefox-78.0.1-lp184.108.40.206.x86_64 MozillaFirefox-branding-openSUSE-68-lp152.1.1.x86_64
All applications in KDE show up in french as expected, except Firefox, which shows up in english.
What now ?