Crowley also spells it "Jesod" in various places (e.g.: The Vision and the Voice; Konx Om Pax), as does Westcott (another Theosophist as well as one of the founders of the GD). In the latin of the Kabbala Denudata
by Christian Knorr von Rosenroth and others, which is a source text for much Golden Dawn material (and sections of which were translated by Mathers*), also spells it "Jesod":
In THE SECRET TEACHINGS OF ALL AGES
in the section of the Tree of the Sephiroth by Manly P. Hall it is also spelt with a J.
So the spelling does not neccesarily denote a German or a Theosophical source. The letter Yod/Iod/Jod is variously transcribed as Y, I or J.
* Mather in his translation however (the Kabbalah Unveiled) transcribes ISVD as Yesod.