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

<?php
/**
 * config.php
 * Copyright (c) 2017 thegrumpydictator@gmail.com
 *
 * This file is part of Firefly III.
 *
 * Firefly III is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * Firefly III is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with Firefly III.  If not, see <http://www.gnu.org/licenses/>.
 */

declare(strict_types=1);
return [
    'locale'          => 'pl, Polish, polski, pl_PL, pl_PL.utf8, pl_PL.UTF-8',
    'month'           => '%B %Y',
    'month_and_day'   => '%e %B %Y',
    'date_time'       => '%e %B %Y o %T',
    'specific_day'    => '%e %B %Y',
    'week_in_year'    => 'Tydzień %W, %Y',
    'quarter_of_year' => '%B %Y',
    'year'            => '%Y',
    'half_year'       => '%B %Y',

];