I'm trying to make a plugin in Craft 3 but want it to be possible to set different settings for each separate site or locale. How would I go about doing this?
OP updated the question to specify Craft 3, so this is no longer relevant.
You can define a separate value for each locale, as described in the localisation documentation:
<?php return [ 'flagName' => [ 'de' => 'Bundesflagge', 'fr' => 'Tricolore', ], ];