Adjectives that are derived from proper nouns are called proper adjectives.
They tend to drink their tea with gloves on and one pinky extended.
"After becoming obsessed with StarCraft, Brett began taking Korean language classes."
In this example, Korean is a proper adjective (derived from the proper noun, Korea) that modifies language.
StarCraft, a real-time strategy video game, is tremendously popular in South Korea. Players compete professionally, earn sponsorships, and even go head-to-head in televised tournaments. In the United States, the Air Force has used it as part of their training program.
"Fern hunting was a popular hobby in Victorian England."
Here, Victorian is a proper adjective that modifies England and refers to the period of time from 1837 to 1901 during which Queen Victoria reigned supreme. People were crazy about plants during the Victorian era, especially ferns, and would go on fern hunting expeditions to the European continent and Asia, in hopes of bringing home a thrilling specimen. So next time your dad suggests taking a family vacation to some dusty old museum, remember: it could always be worse.
"The best part about that Jewish wedding last weekend was when we lifted up the bride and groom during the hora."
Mazel Tov! In this sentence, Jewish is a proper adjective that modifies wedding.