Poland has been a "Catholic country" since its very beginning as the nation, ever since Mieszko was baptised in 966,
When Mieszko got baptised, there was no definite division between Catholicism, Orthodox Christianity and Protestantism. He was baptised simply as a Christian.
and nothing has changed ever since
A lot has changed: there was the Great Schism, Reformation, Union of Brest, the territory of Poland changed and started incorporating lands with Orthodox population (still, around Białystok there's a lot of Orthodox Christians), assimilated Jews started speaking Polish and contributing to Polish culture (like Tuwim), not Jewish (like Singer). A lot of stuff
happened since 966...
Are you talking about the same multicultural Poland in both sentences?
How is the Polish eagle representative of Ukrainians, Latvians or Germans?
The legend is about founding the first Polish town, Gniezno - out of which the state later emerged. The Eagle is representative of the land and the state, so it can be representative of every Polish citizen.