OK
Finally, the solution (which worked for me) is as follows:
gettext relies on system locales.
We therefore need to add the languages used at system level.
I'd forgotten this ‘little’ detail since the last time...
For example, for French
1/ Find the installed locales
locale -a
2/ Add French
locale-gen fr_FR
3/ Update the locales
update-locale
4/ Restart the php site container
I also restarted the server's ‘Application’ role
