I read on Wikipedia that the first version of TeX was written in SAIL. However, does anyone know what language later versions (i.e the current one) are implemented in?
TeX was rewritten in Pascal. Actually, using a literate programming system called WEB, invented by Knuth himself. This system uses Pascal for the programming and it uses TeX for the documentation.
The source code of TeX was published as a book: http://www.amazon.com/Computers-Typesetting-B-TeX-Program/dp/0201134373.
TeX is written in WEB (a literate programming language created by D. E. Knuth).
Originally, WEB source was translated into Pascal (a subset of Pascal) via
tangle to produce executable and translated into TeX via
weave to produce documentation of the code. Both programs have themselves been written in WEB.
Today, TeXLive uses
web2c to translate directly WEB source into C... via the Pascal source produced by tangle (
web2c is not a Pascal to C translator).
MikTeX must use a similar system to produce C/C++.