Simplify the following text for a Grade 9 native English-speaking person.
Original: The Free City of Danzig was a semi-autonomous city-state that existed between 1920 and 1939 , consisting of the Baltic Sea port of Danzig ( today GdaÅ sk ) and surrounding areas .
Simplified: The Free City of Danzig ( German : Freie Stadt Danzig ; Polish : Wolne Miasto GdaÅ sk ) was an self-governing port on the Baltic Sea port and a city-state .