Locale not supported by C library. Using the fallback 'C' locale.

$ gnome-terminal
# Locale not supported by C library.
# 	Using the fallback 'C' locale.

$ locale -a
locale: Cannot set LC_MESSAGES to default locale: No such file or directory
locale: Cannot set LC_COLLATE to default locale: No such file or directory
C
POSIX
en_US.utf8
ru_RU.utf8

$ cat /etc/locale.conf
LANG=ru_RU.UTF-8

locale

env

$ locale
locale: Cannot set LC_MESSAGES to default locale: No such file or directory
locale: Cannot set LC_ALL to default locale: No such file or directory
LANG=ru_KZ.UTF-8
LC_CTYPE=en_US.UTF-8
LC_NUMERIC="ru_KZ.UTF-8"
LC_TIME="ru_KZ.UTF-8"
LC_COLLATE="ru_KZ.UTF-8"
LC_MONETARY="ru_KZ.UTF-8"
LC_MESSAGES="ru_KZ.UTF-8"
LC_PAPER="ru_KZ.UTF-8"
LC_NAME="ru_KZ.UTF-8"
LC_ADDRESS="ru_KZ.UTF-8"
LC_TELEPHONE="ru_KZ.UTF-8"
LC_MEASUREMENT="ru_KZ.UTF-8"
LC_IDENTIFICATION="ru_KZ.UTF-8"
LC_ALL=

$ env
SHELL=/bin/bash
SESSION_MANAGER=local/arch:@/tmp/.ICE-unix/661,unix/arch:/tmp/.ICE-unix/661
WINDOWID=39845891
COLORTERM=truecolor
XDG_SESSION_PATH=/org/freedesktop/DisplayManager/Session1
LANGUAGE=
DESKTOP_SESSION=/usr/share/xsessions/plasma
GTK_RC_FILES=/etc/gtk/gtkrc:/home/recursive_cat/.gtkrc:/home/recursive_cat/.config/gtkrc
XCURSOR_SIZE=24
GTK_MODULES=canberra-gtk-module
XDG_SEAT=seat0
PWD=/home/recursive_cat/Documents/Python/Telegram/ped
LOGNAME=recursive_cat
XDG_SESSION_DESKTOP=KDE
XDG_SESSION_TYPE=x11
XAUTHORITY=/home/recursive_cat/.Xauthority
MOTD_SHOWN=pam
GTK2_RC_FILES=/etc/gtk-2.0/gtkrc:/home/recursive_cat/.gtkrc-2.0:/home/recursive_cat/.config/gtkrc-2.0
HOME=/home/recursive_cat
LANG=ru_KZ.UTF-8
XDG_CURRENT_DESKTOP=KDE
VTE_VERSION=6003
XDG_SEAT_PATH=/org/freedesktop/DisplayManager/Seat0
KDE_SESSION_UID=1000
XDG_SESSION_CLASS=user
TERM=xterm-256color
USER=recursive_cat
KDE_SESSION_VERSION=5
PAM_KWALLET5_LOGIN=/run/user/1000/kwallet5.socket
DISPLAY=:0
SHLVL=1
XDG_VTNR=1
XDG_SESSION_ID=2
LC_CTYPE=en_US.UTF-8
XDG_RUNTIME_DIR=/run/user/1000
QT_AUTO_SCREEN_SCALE_FACTOR=0
XCURSOR_THEME=breeze_cursors
XDG_DATA_DIRS=/home/recursive_cat/.local/share/flatpak/exports/share:/var/lib/flatpak/exports/share:/usr/local/share:/usr/share
KDE_FULL_SESSION=true
PATH=/usr/local/sbin:/usr/local/bin:/usr/bin:/var/lib/flatpak/exports/bin:/usr/bin/site_perl:/usr/bin/vendor_perl:/usr/bin/core_perl
DBUS_SESSION_BUS_ADDRESS=unix:path=/run/user/1000/bus
KDE_APPLICATIONS_AS_SCOPE=1
MAIL=/var/spool/mail/recursive_cat
_=/usr/bin/env
OLDPWD=/home/recursive_cat

 grep -v "^#\|^$" /etc/locale.gen

en_US.UTF-8 UTF-8
ru_RU.UTF-8 UTF-8

ConnorDash
locale
locale: Cannot set LC_MESSAGES to default locale: No such file or directory
locale: Cannot set LC_ALL to default locale: No such file or directory
LANG=ru_KZ.UTF-8

$  grep -v "^#\|^$" /etc/locale.gen
en_US.UTF-8 UTF-8
ru_RU.UTF-8 UTF-8

ConnorDash
LANG=ru_KZ.UTF-8

У вас не сгенерирована такая локаль ru_KZ.UTF-8

Ее собственно в арче и нет, есть KZ.UTF-8 UTF-8
Перенастройте локаль, гном напакостил скорее всего

ConnorDash
Но этот LANG=ru_KZ.UTF-8 нигде не фигурирует

ConnorDash
env
LANG=ru_KZ.UTF-8