Modern German belongs, with English, to the West Germanic branch of the Indo-European family, and it carries one of the densest bodies of original religious and esoteric writing in Europe.
The Reformation made it a scriptural language. Luther's Bible did not only translate; it fixed a written standard that later German religious writing was composed in. Alongside it ran a mystical tradition with a vocabulary of its own, Meister Eckhart preaching in Middle High German, and Jakob Bohme writing in the early seventeenth century in a German so idiosyncratic that his translators still disagree over it.
The esoteric line is equally German rather than translated. Paracelsus wrote and lectured in German on alchemy and medicine, and the names of the elemental spirits, sylphs, undines, salamanders and gnomes, come out of that milieu rather than from a classical source. The Rosicrucian manifestos of the early seventeenth century appeared in German. Guido von List published his Armanen runic system in German in 1906 and 1908, and it passed from there into the volkisch and later the Nazi-era occult milieu in the same language.
For all of that material the question of what language it was written in has one answer, and it is not the language a modern reader happens to be holding it in. A German original and an English translation of it are two texts with two histories, and only one of them is evidence of what its author actually wrote.