chrześcijaninem is almost unprononciable. Duo should add a fonction which prononces the words slowlier in order we can better understand and repeat them
Not specific to this but Polish does not capitalize a lot of proper nouns, does it? I look for patterns but I would guess that more goes uncapitalized than capitalized as it would be in English.
For example titles (of songs, movies, books) in English capitalize almost every word apart from prepositions and articles, but in Polish they capitalize only the first word and proper nouns.