You may want to set an environment variable for a single command, to test out a translation without having to change your system-wide language settings (e.g. LANG=pt_BR.UTF-8) or to enable debug flags (e.g. G_MESSAGES_DEBUG=all).

To do that, simply prepend the variable attribution to the command like so:

VARIABLE=value command

For flatpaks, which run on their own isolated environment, you must use the --env flag:

flatpak run com.example.Application --env VARIABLE=value

Sources: [1] [2] [3]