If you are using KDE, I’d stick to the KDE-way of doing things through their System Settings.
This is actually a Manjaro setting section (it’s a separate app in XFCE)
/etc/locale.gen is managed via: KDE System Settings > Locale
There are 2 language variables, plus LC variables. Take a look at your environment:
declare -p ${!LANG@} ${!LC@}
There is also, KDE System Settings > Regional Settings > Language
This seems to update the file ~/.config/plasma-localerc.
imho, deleting a file is always the last resort. Always backup before deleting.
I have a file ~/.config/plasma-locale-settings.sh that says “do not edit, generated script… generated by kcmshell5 formats”, which is the same as KDE System Settings > Regional Settings > Formats.
man man says:
This package supports manual pages in multiple languages, controlled by your locale. If your system did not set this up for you automatically, then you may needs to set $LC_MESSAGES, $LANG, or another…
You may need to logout/in to get the new environment. This seemed to take a long time. Resist hitting reset 
If you do a man -d man (second man can be replaced), the output will show what locales man is looking for. (For example, I added Deutsch via Settings > Regional Settings > Language and I promoted it to the default (hover over the language and icons appear on the right). The debug output showed:
checking for locale de
checking for locale en_US
checking for locale en_US.UTF.8
This updated file .config/plasma-localerc, which set
[Translations]
LANGUAGE=de:en_US
locale -a or localectl list-locales didn’t change. They only contained en_US.UTF.8 in my case.
Bottom-line
Go to Settings > Regional Settings > Language.
Set your languages. The first one is the default.
Apply.
Logoff/on (wait…).
Double check: cat ~/.config/plasma-localrc