This is the 10435th most frequent Polish word.
czeski
"Czeski" means "Czech" in English.
Here "czeski" is used as an adjective describing the noun "język" (language), indicating something related to the Czech language.
Czeski język jest bardzo interesujący.
The Czech language is very interesting.
In this sentence "Czeszka" is a feminine noun derived from "czeski," referring to a woman of Czech origin.
Moja przyjaciółka jest Czeszka.
My friend is Czech.
Here "czeskim" is a locative form of the adjective "czeski," agreeing with the noun "sklepie" (shop) it describes.