環境

OS: CentOS Linux release 8.2.2004 (オンプレ/実機)
Kernel: 4.18.0-193.6.3.el8_2.x86_64
Locale: System Locale: LANG=en_US.UTF-8

現象

[root@nbs2 ~]# dnf check
Failed to set locale, defaulting to C.UTF-8
[root@nbs2 ~]#

原因

言語パッケージがインストールされていない

[root@nbs2 ~]# localectl status
   System Locale: LANG=en_US.UTF-8
       VC Keymap: jp
      X11 Layout: jp
[root@nbs2 ~]#

[root@nbs2 ~]# localectl list-locales
C.utf8
ja_JP.eucjp
ja_JP.utf8
[root@nbs2 ~]#

[root@nbs2 ~]# dnf info langpacks-ja
Failed to set locale, defaulting to C.UTF-8
Last metadata expiration check: 0:50:25 ago on Sun Jun 21 14:39:30 2020.
Installed Packages
Name         : langpacks-ja
Version      : 1.0
Release      : 12.el8
Architecture : noarch
Size         : 402
Source       : langpacks-1.0-12.el8.src.rpm
Repository   : @System
From repo    : AppStream
Summary      : Japanese langpacks meta-package
License      : GPLv2+
Description  : This package provides Japanese langpacks meta-package.

[root@nbs2 ~]#

[root@nbs2 ~]# dnf info langpacks-en
Failed to set locale, defaulting to C.UTF-8
Last metadata expiration check: 0:50:32 ago on Sun Jun 21 14:39:30 2020.
Available Packages
Name         : langpacks-en
Version      : 1.0
Release      : 12.el8
Architecture : noarch
Size         : 9.6 k
Source       : langpacks-1.0-12.el8.src.rpm
Repository   : AppStream
Summary      : English langpacks meta-package
License      : GPLv2+
Description  : This package provides English langpacks meta-package.

[root@nbs2 ~]#

処置

参考

RHEL8でコマンド実行時に Failed to set locale, defaulting to C と表示される - Qiita
RHEL 8 の導入における検討事項 Red Hat Enterprise Linux 8 | Red Hat Customer Portal

内容

dnf -y install langpacks-en

dnf -y install langpacks-en 実行すれば解決
場合によっちゃ依存関係で他のパッケージも必要になるけど dnf(yumはCentos8では非推奨) 使えば一瞬

[root@nbs2 ~]# dnf list langpacks-en
Failed to set locale, defaulting to C.UTF-8
Last metadata expiration check: 0:56:16 ago on Sun Jun 21 14:39:30 2020.
Available Packages
langpacks-en.noarch                         　     1.0-12.el8                              AppStream
[root@nbs2 ~]#

Available Packages になっているのでインストールされていない。

[root@nbs2 ~]# dnf -y install langpacks-en
Failed to set locale, defaulting to C.UTF-8
Last metadata expiration check: 0:56:27 ago on Sun Jun 21 14:39:30 2020.
Dependencies resolved.
====================================================================================================
 Package                             Architecture        Version             Repository        Size
====================================================================================================
Installing:
 langpacks-en                        noarch              1.0-12.el8          AppStream        9.6 k
Installing weak dependencies:
 glibc-langpack-en                   x86_64              2.28-101.el8        BaseOS           821 k

Transaction Summary
====================================================================================================
Install  2 Packages

Total download size: 831 k
Installed size: 6.0 M
Downloading Packages:
(1/2): langpacks-en-1.0-12.el8.noarch.rpm                           103 kB/s | 9.6 kB     00:00
(2/2): glibc-langpack-en-2.28-101.el8.x86_64.rpm                    1.2 MB/s | 821 kB     00:00
----------------------------------------------------------------------------------------------------
Total                                                                400 kB/s | 831 kB     00:02
Running transaction check
Transaction check succeeded.
Running transaction test
Transaction test succeeded.
Running transaction
  Preparing        :                                                                            1/1
  Installing       : glibc-langpack-en-2.28-101.el8.x86_64                                      1/2
  Installing       : langpacks-en-1.0-12.el8.noarch                                             2/2
  Running scriptlet: langpacks-en-1.0-12.el8.noarch                                             2/2
  Verifying        : langpacks-en-1.0-12.el8.noarch                                             1/2
  Verifying        : glibc-langpack-en-2.28-101.el8.x86_64                                      2/2
Installed products updated.

Installed:
  glibc-langpack-en-2.28-101.el8.x86_64                              langpacks-en-1.0-12.el8.noarch

Complete!
[root@nbs2 ~]#

Installed と Complete! が表示されればOK
不安だったらrpm -q langpacks-en 叩けばいい
インストールされてなければpackage langpacks-en is not installedと表示される

langpacks-en

[root@nbs2 ~]# rpm -q langpacks-en
langpacks-en-1.0-12.el8.noarch
[root@nbs2 ~]#

langpacks-eu

[root@nbs2 ~]# rpm -q langpacks-eu
package langpacks-eu is not installed
[root@nbs2 ~]#

[root@nbs2 ~]# localectl status
   System Locale: LANG=en_US.UTF-8
       VC Keymap: jp
      X11 Layout: jp

[root@nbs2 ~]# localectl list-locales
C.utf8
en_AG
en_AU
en_AU.utf8
en_BW
en_BW.utf8
en_CA
en_CA.utf8
en_DK
en_DK.utf8
en_GB
en_GB.iso885915
en_GB.utf8
en_HK
en_HK.utf8
en_IE
en_IE.utf8
en_IE@euro
en_IL
en_IN
en_NG
en_NZ
en_NZ.utf8
en_PH
en_PH.utf8
en_SC.utf8
en_SG
en_SG.utf8
en_US
en_US.iso885915
en_US.utf8
en_ZA
en_ZA.utf8
en_ZM
en_ZW
en_ZW.utf8
ja_JP.eucjp
ja_JP.utf8
[root@nbs2 ~]#

en_* が増えた

[root@nbs2 ~]# dnf check
[root@nbs2 ~]#

インストールしたら Failed to set locale, defaulting to C.UTF-8 が出なくなった