Plugin settings per site/locale
Edit

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:

&lt;?php

return [
    'flagName' =&gt; [
        'de' =&gt; 'Bundesflagge',
        'fr' =&gt; 'Tricolore',
    ],
];