The German loan word Marmelade for any kind of fruit preserve predates the first mentioning of marmalade in English literature by about a century.
The real origin can be found in Greek, passed to us via Romance Languages, in particular Portuguese and Spanish.
Yet, the British influence forced all EU citizens to follow their1 definition of marmalade as a fruit preserve exclusively made from citrus fruits.
Even the above linked Wikipedia article concedes that, quote:
Historically, as mentioned hereinafter, the term was used more often in other senses than just citrus conserves.
Which makes a little bit of sense once you realize that the word for quinces in Greek is the origin of the term in the first place.
So since the queer British meaning of the word marmalade was forced upon all Europeans, will a Brexit mean a return to sanity in this respect? After all Brits would be the first to criticize other people for queer uses of words.
And if we had to agree on a type or category of fruit allowed in marmalade2, let it be quinces. After all the Portuguese marmelada, commonly quoted as the origin for the English word marmalade, is used exclusively for quince jam, pardon me: quince marmalade. In Spanish it means any jam and in Italian and German it means any jam or marmalade.