Let $G$ be an adjoint Chevalley group. Are all of $G(\mathbb Z)$'s finite-index subgroups congruence subgroups?  

I read a theorem that states: When $G$ is the universal Chevalley group and it's not of type $\operatorname{SL}\_{2}$ every finite index subgroup of $G(\mathbb Z)$ is a congruence subgroup (in [Matsumoto - Sur les sous-groupes arithmétiques des groupes semi-simple déployés](http://www.numdam.org/article/ASENS_1969_4_2_1_1_0.pdf)).


I've been looking for sources and results on that topic that I can learn and quote form, unfortunately, all I could find was in French.



For $G$ an adjoint Chevalley group, are all of $G(\mathbb Z)$'s finite-index subgroups congruence subgroups?
Chapter VI of my old Springer Lecture Notes in Mathematics 789 *Arithmetic Groups* (1980) is in English and gives a version of H. Matsumoto's and C. Moore's arguments for the subgroups of $\operatorname{SL}\_n(\mathbb{Z})$ of finite index when $n \geq 3$. But this account also gives some clues to the general case, where even for simply connected groups there are exceptions for *imaginary* fields.


These lecture notes are somewhat eclectic and involve an expanded version of earlier lectures I gave at the Courant Institute in the early 1970s.
There is also a list of references up to 1980.


But the advice to learn some mathematical French is well justified, since so much important literature is in French. On the other hand, the earlier paper by Bass, Milnor, and Serre is in English but quite limited. Later work by G. Prasad and M. Raghunathan provides more technical details about the general case over an arbitrary (say) number field.


