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

<?php
declare(strict_types=1);

/**
 * config.php
 * Copyright (C) 2016 thegrumpydictator@gmail.com
 *
 * This software may be modified and distributed under the terms of the
 * Creative Commons Attribution-ShareAlike 4.0 International License.
 *
 * See the LICENSE file for details.
 */

return [
    'locale'          => 'es, Spanish, es_ES, es_ES.utf8, es_ES.UTF-8',
    'month'           => '%B %Y',
    'month_and_day'   => '%B %e, %Y',
    'date_time'       => '%B %e, %Y, @ %T',
    'specific_day'    => '%e %B %Y',
    'week_in_year'    => 'Semana %W, %Y',
    'quarter_of_year' => '%B %Y',
    'year'            => '%Y',
    'half_year'       => '%B %Y',

];