I find it fascinating this name is completely unknown in Poland, while it's relatively common in other Slavic countries, such as Russia, Czechia, Slovakia and Croatia.
Denis Olegovich Istomin (1986-) is an Uzbek professional tennis player of Russian descent. He has won two singles titles (at the 2015 Aegon Open Nottingham and 2017 Chengdu Open) and achieved a career-high singles ranking of world No. 33 in August 2012. In January 2017, he defeated defending champion Novak Djokovic in the second round of the Australian Open. He won a gold medal at 2018 Asian Games and will represent Uzbekistan at the 2020 Summer Olympics in Tokyo.
Borne by Kazakhstani figure skater Denis Ten (Денис Тен, 1993-2018), who was a member of the Korean minority in Kazakhstan (also called the Koryo-saram or Koryoin).
Umm St. Denis IS one of the patron saints of France. He and Joan of Arc are the two, not Therese de Lisieux. I actually like this name pronounced the French way: de-Nee. Sounds cool. :)
Also Polish: https://www.behindthename.com/name/denis/top/poland
https://dane.gov.pl/pl/dataset/1667,lista-imion-wystepujacych-w-rejestrze-pesel-osoby-zyjace/resource/54115/table?page=1&per_page=20&q=Denis&sort=col1