oneThis plural form is used for numbers like: 1
Every %1$d week
otherThis plural form is used for numbers like: everything else
Every %1$d weeks
Co %1$d tydzień
Co %1$d tygodnie
Co %1$d tygodni
Every %1$d week
Every %1$d weeks
Every %1$d week
Every %1$d weeks
Co %1$d tydzień
Co %1$d tygodnie
Co %1$d tygodni
%1$d - number, %1$d may be skipped for singular if translation looks better.