Hello alessandro,
Zettelkasten is a natively a german program. It is based on the "Zettelkasten" (slip box) of Niklas Luhmann, a german sociologist, who had a really big physical analogue "Zettelkasten".
Here are some english links:
http://takingnotenow.blogspot.com/20...telkasten.html
In this blog there are also more entries to Luhmanns Zettelkasten; use tag "Zettelkasten".
http://pileofindexcards.org/blog/200...kasten-method/
Video about Niklas Luhmann's Zettelkasten
http://japanologie.arts.kuleuven.be/...+knowledge.pdf
Here you find some information about Luhmann's Zettelkasten translate in english.
Greetings, Maria!