In Slavic cultures Kupala was a goddess of beauty and love. She was celebrated/honored/worshiped during the summer solstice. After Christianity took over in Slavic countries her cult was suppressed but it still survived, it was just combined with Christianity (the feast of St. John the Baptist which is also around the Summer Solstice), but many of the original superstitions and traditions survived. These celebrations are still held in some Slavic countries, mainly Russian, Ukraine, Poland, and Belarus. Although they used to be much more common decades ago. But anyways on the night of the festival girls make wreaths out of flowers and tell fortunes about love, dance around fire, sing songs, ect. They then make boards to hold candles and wrap their flower wreaths around the candles and go to a body of water (usually a small one) and send their boards with the wreath and candle into the water hoping it will float in a straight line.. also the boys of the village try to catch these boards with one of the girls wreaths/candles on it and if he does this signifies that the two are destined to be together.

So in this song Kupalanka is a diminutive either for the goddess Kupala or a woman/girl partaking in her feast/summer solstice celebrations of Kupala. There is definitely no English translation of this word though.

I tried to write and explanation above. I hope it makes sense. :)