Which European national flag does not have a cross on it?

The national flag of Slovenia does not have a cross on it.

The national flag of Slovenia features three equal horizontal bands of white (top), blue, and red, with the Slovene coat of arms located in the upper hoist side of the flag centered in the white and blue bands. The coat of arms is a shield with the image of Mount Triglav, Slovenia's highest peak, in white against a blue background at the center; beneath it are two wavy blue lines representing the Adriatic Sea and local rivers, and above it are three six-pointed golden stars arranged in an inverted triangle which are taken from the coat of arms of the Counts of Celje, the great Slovene dynastic house of the late 14th and early 15th centuries.

The flag's colors are considered to be Pan-Slavic, but they actually come from the medieval coat of arms of the Duchy of Carniola, consisting of a blue eagle on a white background with a red-and-gold crescent. The existing Slovene tricolor was raised for the first time in history during the Revolution of 1848 by the Slovene Romantic nationalist activist and poet Lovro Toman on 7 April 1848, in Ljubljana, in response to a German flag which was raised on top of Ljubljana Castle.