What does Sjaellandia mean in Latin?

Sjaellandia, a proper noun, means “Zealand (an island of Denmark)”.

What is the genitive of Sjaellandia?

The genitive singular is Sjaellandiae. Sjaellandia is feminine, first declension.

In classical Latin, [sjae̯lˈlan.di.a]; in ecclesiastical (church) Latin, [sjelˈlan.di.a].

proper noun · femininefirst declension

Sjaellandia f sg (genitive Sjaellandiae)

Classical[sjae̯lˈlan.di.a]Ecclesiastical[sjelˈlan.di.a]

femininesingularZealand (an island of Denmark)
Itaque magnam partem littorum Daniae a Skagen ad Hafniam visitavi; ostrearia vero inter Fredrikshavn^([sic]) et Skagen, sinus in Sjaellandia, qui Issefjord nominatur, et fretum Öresund maximum mihi specierum numerum praebuerunt.I have visited a large part of the coast of Denmark from Skagen to Copenhagen; but the oyster beds between Frederikshavn and Skagen, the bay in Zealand, which is called Issefjord, and the strait of Öresund have furnished me with the greatest number of species.1843, Anders Sandøe Ørsted, Annulatorum danicorum conspectus, Praemonenda:

From Danish Sjælland + Latin -ia.