This is the sort of search that AI would make a lot easier. Right now searching for the first use of a term in English isn't the sort of thing Google does well.

It's worth noting that 'Zettel', a collection of Wittgenstein's remarks, was published in 1967 on English. See https://archive.org/details/zettel0000witt The title doesn't use the full word 'Zettelkasten' but that's the concept.

Just for fun I did a bit more searching and found https://ablinger.mur.at/zettelkasten.html which was created no earlier than July 22, 2000 (based on it being authored for Mozilla 4.74 but probably not much later. Individual entries are dated and there are many prior to 2007, eg. https://ablinger.mur.at/zettel_rhythmen-zeichnen.html