Katja is very rarely used in Poland and pronounced KAHT-yah.
― Anonymous User 1/12/2018
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As Caprice said, Katja is also used in Finland. It is rather common too, with over 18,000 occurrences (includes middle names) according to the Finnish Population Register Centre. Katja's name day is on 25th of November, same as other Finnish names related to Katherine. [noted -ed]
It's used in the Netherlands as well. A friend of mine bears this name and I like it. Especially because it had the Dutch word "kat" in it which means "cat".