Localizing the vnstat PHP frontend into Polish is very simple - just create a file called pl.php with the content:

<?php

// sidebar labels
$L['summary'] = 'suma';
$L['hours'] = 'godziny';
$L['days'] = 'dni';
$L['months'] = 'miesiace';

// main table headers
$L['Summary'] = 'Suma';
$L['Top 10 days'] = 'Top 10 dni';
$L['Last 24 hours'] = 'Ostatnie 24 godziny';
$L['Last 30 days'] = 'Ostatnie 30 dni';
$L['Last 12 months'] = 'Ostanie 12 miesiecy';

// traffic table columns
$L['In'] = 'Pobieranie';
$L['Out'] = 'Udostepnanie';
$L['Total'] = 'W sumie';

// summary rows
$L['This hour'] = 'Godzina';
$L['This day'] = 'Dzien';
$L['This month'] = 'Miesiac';
$L['All time'] = 'Caly czas';

// graph text
$L['Traffic data for'] = 'Statystyki dla: ';
$L['bytes in'] = 'Dane wchodzace';
$L['bytes out'] = 'Dane wychodzace';

// date formats
$L['datefmt_days'] = '%d %B';
$L['datefmt_days_img'] = '%d';
$L['datefmt_months'] = '%B %Y';
$L['datefmt_months_img'] = '%b';
$L['datefmt_hours'] = '%l%P';
$L['datefmt_hours_img'] = '%l';
$L['datefmt_top'] = '%d %B %Y';

After creating the pl.php file, we move it to the directory path_to_vnstat_php_frontend/lang, then in the config.php file we change the variable $language = 'en'; to $language = 'pl'; .