I want to learn the proof of the following theorem by Siegel. The statement of the theorem is taken from "Symmetric bilinear forms" by Milnor and Husemoller (pp. 44). They say that the proof is due to C.L. Seigel and appears in Gesammelte Abhandlungen I, Springer 1966. The problem is that I do not know German. Also, given that I am a theoretical computer scientist, I would like a proof which is as simple as possible.
Let $X$ be a positive definite bilinear form space of rank $\geq 2$ over $Z$. For each integer $k$ let $r_X(k)$ denote the number of distinct elements $x \in X$ satisfying the equation $x . x = k$. If the genus of $X$ contains only one isomorphism class then for $f(x_1,x_2, \dots, x_k) = x_1^2+x_2^2+\dots+x_n^2$
$r_x(k) = \epsilon \prod_{p=2,3,\dots,\infty} Df_p^{-1}(k)$
for every integer $k \neq 0$, where the coefficient $\epsilon$ is defined to be either $\frac{1}{2}$ or $1$ according as $n=2$ or $n > 2$.
Any pointers to a book/online report where the proof appears will be very helpful.