firefly-iii/resources/lang/nl_NL/config.php

<?php

return [
    'locale'          => 'nl, Dutch, nl_NL, nl_NL.utf8',
    'month'           => '%B %Y',
    'month_and_day'   => '%e %B %Y',
    'specific_day'    => '%e %B %Y',
    'week_in_year'    => 'week %W, %Y',
    'quarter_of_year' => '%B %Y',
    'year'            => '%Y',
    'half_year'       => '%B %Y',

];