By default, the rights "configure", "configure-all" and "extensions" are given to bureaucrats. We can simply install the extension, then override those settings with

$wgGroupPermissions['bureaucrat']['configure'] = false;
$wgGroupPermissions['bureaucrat']['configure-all'] = false;
$wgGroupPermissions['bureaucrat']['extensions'] = false;

and then no harm can come from it. (Special:ViewConfig will still be available for sysops)

But the fact that this extension doesn't handle ALL configuration settings makes me think we might need a more robust solution. Would it make sense to have a DefaultSettings.i18n.php file?

On the other hand, the unsupported configuration settings are just a handful: [[mw:Extension:Configure#Special:Configure]]. Maybe the extension's i18n file can include those just for completeness.

Which of these choices would be preferable?

(In reply to comment #1)

Which of these choices would be preferable?

I think that it doesn't matter and should be a separate bug against the extension. If the community agrees on the fact that it's useful to have the possibility to use such messages on wiki and no technical obstacle is found to enabling it "read only", such imperfections can be sorted out later/independently.
In the discussion on wiki nobody opposed; this bug has not yet had comments on security problems or better alternatives or lack thereof, maybe the devs who didn't like the idea much at first will find out that there's no reason not to do it.

(In reply to comment #2)

(In reply to comment #1)

Which of these choices would be preferable?

I think that it doesn't matter and should be a separate bug against the
extension. If the community agrees on the fact that it's useful to have the
possibility to use such messages on wiki and no technical obstacle is found
to enabling it "read only", such imperfections can be sorted out
later/independently.

Agreed, let's focus the discussion on getting the extension deployed.

maybe the devs who didn't like the idea much at first will find out
that there's no reason not to do it.

Who are the devs who didn't like the idea? Maybe we should poke them towards commenting on this bug.