It's difficult to say,but I try to explain.
Firstly,club "Legia" from Warsaw was founded by members of Polish Legions in 1916 when Poland fight for independence.And name "Legions" or "Legionnaires" exists in Roman Empire,so I think that people wanted to find something in common with it.Roman Empire was also big and powerful,so we use it to show our strentgh and bravery like Roman Empire warriors.Also graffities with knights and cavaliers were use to show it too.
Poland is a Catholic country where over 95% people belong to Roman Catholic church and we don't use cyrylic alphabet,so we have more in common with "western culture".Pope John Paul 2 was from Poland too.Books like "Quo Vadis" written by Henryk Sienkiewicz(I don't know if You know this,it is about Christianity during Roman Empire times) are treated like a national masterpiece of art.
Many people try to explain feelings using Latin sentences from Ancient times because they are very "deep".It is still very popular in Poland and people generally know their meanings,so You can see, for example "Semper fidelis".Also many fan groups(ultras or hooligans) named themselves in Latin for example "Persona non grata" (GKS Katowice),"Torcida" (GÃ³rnik Zabrze) or "Barra Bravas" (ZagÅ‚Ä™bie Sosnowiec).
Many club names are Latin also, for example "Cracovia" or "Resovia RzeszÃ³w.
Graffities are are also very often inspired by our history(like Legia graffities about Warsaw Uprising in 1.08.1944).We remember it.
You can see on Zaglebie Sosnowiec graffities knights with inscription "Sarmaci"-there were a popular myth in 18th century in Poland, which told that Poles came from brave and valiant nation called Sarmatians.Now we know that it isn't true and Poles are Slavs,but artists use it to show Polish power,bravely and patriotism.
We are proud of our history,we were fought for God,homeland and honour.This mentality still exist in my country so many foreigners often say that Polish people are patriots and I think that it's true.
Sorry for my english,I'm beginner Â