add-apt-repository is broken with non-UTF-8 locales

I'm trying to install PHP version 7.4 with NGINX, and I'm getting the following error when trying to add a repository:

sudo add-apt-repository ppa:USER/php
Co-installable PHP versions: PHP 5.6, PHP 7.x and most requested extensions are included. Only Supported Versions of PHP (http://php.net/supported-versions.php) for Supported Ubuntu Releases (https://wiki.ubuntu.com/Releases) are provided. Don't ask for end-of-life PHP versions or Ubuntu release, they won't be provided.

Debian oldstable and stable packages are provided as well: https://deb.sury.org/#debian-dpa

You can get more information about the packages at https://deb.sury.org

IMPORTANT: The <foo>-backports is now required on older Ubuntu releases.</foo>

BUGS&FEATURES: This PPA now has a issue tracker:
https://deb.sury.org/#bug-reporting

CAVEATS:

If you are using php-gearman, you need to add ppa:USER/pkg-gearman
If you are using apache2, you are advised to add ppa:USER/apache2
If you are using nginx, you are advised to add ppa:USER/nginx-mainline
   or ppa:USER/nginx
PLEASE READ: If you like my work and want to give me a little motivation, please consider donating regularly: https://donate.sury.org/

WARNING: add-apt-repository is broken with non-UTF-8 locales, see
https://github.com/oerdnj/deb.sury.org/issues/56 for workaround:

LC_ALL=C.UTF-8 add-apt-repository ppa:USER/php
More info: https://launchpad.net/~USER/+archive/ubuntu/php
Press [ENTER] to continue or ctrl-c to cancel adding it

Exception in thread Thread-1:
Traceback (most recent call last):
File "/usr/lib/python3.7/threading.py", line 917, in _bootstrap_inner
self.run()
File "/usr/lib/python3.7/threading.py", line 865, in run
self._target(*self._args, **self._kwargs)
File "/usr/lib/python3/dist-packages/softwareproperties/SoftwareProperties.py", line 688, in addkey_func
func(**kwargs)
File "/usr/lib/python3/dist-packages/softwareproperties/ppa.py", line 386, in add_key
return apsk.add_ppa_signing_key()
File "/usr/lib/python3/dist-packages/softwareproperties/ppa.py", line 252, in add_ppa_signing_key
tmp_keyring, tmp_secret_keyring, signing_key_fingerprint, tmp_keyring_dir):
File "/usr/lib/python3/dist-packages/softwareproperties/ppa.py", line 181, in _recv_key
"--recv", signing_key_fingerprint,
File "/usr/lib/python3.7/subprocess.py", line 323, in call
with Popen(*popenargs, **kwargs) as p:
File "/usr/lib/python3.7/subprocess.py", line 775, in init
restore_signals, start_new_session)
File "/usr/lib/python3.7/subprocess.py", line 1522, in _execute_child
raise child_exception_type(errno_num, err_msg, err_filename)
FileNotFoundError: [Errno 2] No such file or directory: 'gpg': 'gpg'