Can't set multiple languages for one user
It's currently impossible to tell the OS that we're interested in multiple languages on a single-user system, so that, for example, dnf, or Flatpak, would automatically install the translations, and data sets for those languages, in addition to the main language.
Setting the LANGUAGE
environment variable would allow doing that:
https://www.baeldung.com/linux/locale-environment-variables#5-language
Setting LANGUAGE
to en_GB:fr:pt
in the session would tell the system that I'm interested in all 3 languages/language variants, so that it would automatically install translations and dictionaries.
This might also need changes in accountservices, systemd, gnome-initial-setup, flatpak, etc.