Before load a plugin at Admin Panel just open file named as jquery.countdown.min.js at tools/plugins/start-countdown and then edit labels: ["Years","Months","Weeks","Days","Hours","Minutes","Seconds"],labels1:["Year","Month","Week","Day","Hour","Minute","Second"]
to:
["Lata","Miesiące","Tygodnie","Dni","Godzin","Minut","Sekund"],labels1:["Rok","Miesiąc","Tydzień","Dzień","Godzine","Minute","Sekunde"]
If you already added a plugin then go into your site folder and at tools/plugins/start-countdown you will find such file. Original answer by OtLand user Roni123.