Both Sweden and Sweeden would be more suitable as medieval Dutch forms of Sweden than Zweden, even though medieval Dutch lacked official spelling rules and thus the writing was largely based on sounds (i.e. phonetic). This is because the letter Z was barely used in medieval times. As such, many words and names that are nowadays written with a -z- in Dutch, were written with an -s- in medieval times. For example: the Dutch word 'zwaard' meaning "sword" was written as 'swaert' in Middle Dutch. Another example: the word 'zilver' meaning "silver" was written as 'silver' in both Middle Dutch and Old Dutch. [noted -ed]• https://en.wiktionary.org/wiki/zwaard#Dutch (in English) • https://en.wiktionary.org/wiki/zilver#Dutch (in English)The same sort of principle applies to the European country that is nowadays known as Zweden in Dutch. Another European country to which this applies is Switzerland: it is known as Zwitserland in modern Dutch, but its name would usually be written as Switserland, Switserlandt and Switserlant in medieval times. Also Zwitserlandt and Zwitserlant, but less often.• https://en.wiktionary.org/wiki/Zwitserland#Alternative_forms (in English)This situation basically remained until at least the 17th century. This is why in the Afrikaans language, which is descended from late 17th-century Dutch, the modern names for these two countries are Swede (instead of Zwede) and Switserland (instead of Zwitserland).All in all: even though Zweden would technically not be incorrect as a medieval Dutch name for the country (due to all the possible spellings in medieval times), I would personally be sooner inclined to list the more common Sweden and Sweeden as medieval names for the country. Wiktionary even states that the modern English name for the country ultimately comes from Middle Dutch:• https://en.wiktionary.org/wiki/Sweden#Etymology (in English)Lastly, here are some old books that feature these old Dutch spellings for the country.• Heeren Ambassadeurs van Vranckrijck ende Sweden ("Lord Ambassadors of France and Sweden"): see the title of this book from 1632: https://books.google.de/books?id=ythKAAAAcAAJ&pg=PP3 (in Dutch) • Koninglijcke Brief aen het Rijck Sweden ("Royal Letter to the Kingdom of Sweden"): see page 5 of this book from 1635: https://books.google.de/books?id=tdpKAAAAcAAJ&pg=PP5 (in Dutch) • syn Majesteyt van Sweeden ("His Majesty of Sweden"): see paragraph no. 4 on page 5 of this book from 1659: https://books.google.de/books?id=Ga5OAAAAcAAJ&pg=PA5 (in Dutch) • hare Koninghlijcke Majest. Van Sweeden ende Denemarcken ("Her Royal Majesty of Sweden and Denmark"): see page 30 of this book from 1662: https://books.google.de/books?id=ZaBSSlTBjtIC&pg=PP30 (in Dutch)
• https://en.wiktionary.org/wiki/zwaard#Dutch (in English)
• https://en.wiktionary.org/wiki/zilver#Dutch (in English)
The same sort of principle applies to the European country that is nowadays known as Zweden in Dutch. Another European country to which this applies is Switzerland: it is known as Zwitserland in modern Dutch, but its name would usually be written as Switserland, Switserlandt and Switserlant in medieval times. Also Zwitserlandt and Zwitserlant, but less often.
• https://en.wiktionary.org/wiki/Zwitserland#Alternative_forms (in English)
This situation basically remained until at least the 17th century. This is why in the Afrikaans language, which is descended from late 17th-century Dutch, the modern names for these two countries are Swede (instead of Zwede) and Switserland (instead of Zwitserland).
All in all: even though Zweden would technically not be incorrect as a medieval Dutch name for the country (due to all the possible spellings in medieval times), I would personally be sooner inclined to list the more common Sweden and Sweeden as medieval names for the country. Wiktionary even states that the modern English name for the country ultimately comes from Middle Dutch:
• https://en.wiktionary.org/wiki/Sweden#Etymology (in English)
Lastly, here are some old books that feature these old Dutch spellings for the country.
• Heeren Ambassadeurs van Vranckrijck ende Sweden ("Lord Ambassadors of France and Sweden"): see the title of this book from 1632: https://books.google.de/books?id=ythKAAAAcAAJ&pg=PP3 (in Dutch)
• Koninglijcke Brief aen het Rijck Sweden ("Royal Letter to the Kingdom of Sweden"): see page 5 of this book from 1635: https://books.google.de/books?id=tdpKAAAAcAAJ&pg=PP5 (in Dutch)
• syn Majesteyt van Sweeden ("His Majesty of Sweden"): see paragraph no. 4 on page 5 of this book from 1659: https://books.google.de/books?id=Ga5OAAAAcAAJ&pg=PA5 (in Dutch)
• hare Koninghlijcke Majest. Van Sweeden ende Denemarcken ("Her Royal Majesty of Sweden and Denmark"): see page 30 of this book from 1662: https://books.google.de/books?id=ZaBSSlTBjtIC&pg=PP30 (in Dutch)