Comments for the name Kaja (2)

As a pet form of Karolina, Kaja has only been used in Poland within the last 20 years (or less). Thus, it's fairly modern.

Kája is used as a pet form also in Czechoslovakia.

Also popular as a girls name in Slovenia.

Also a girls name in Croatia. It was fairly common before 1970, and became popular again in the late 1990s.

In Croatia Kaja is almost always thought of as a diminutive of Katarina, but it can be a short form of any name beginning with "Ka" like Karolina.
In fact very rarely it was also used as a masculine name, a variant/nickname of Kajo (Gajo, Gaj) itself the Croatian form of Gaius.