Mateja is from the Slovenian / Croatian Mateja — South Slavic feminine of Matthew. Mateja is one of the Slovenian + Croatian feminine names — central to traditional South Slavic Catholic heritage.
Mateja does not currently appear in the US Social Security Administration's top 1,000 girls' names, so we don't publish a US rank or birth count for it. That says nothing about the name's standing elsewhere in the world — only that it sits outside the ranked US data we rely on.
In Pythagorean numerology the letters of Mateja reduce to 5, The Seeker. This is a traditional interpretive system, not a factual claim about the name.