All the programs in the TeX series (TeX by itself, but also METAFONT and all the auxiliaries) are written in WEB, but to ease the translation, D. E. Knuth has not really written in a flavour of Pascal, but in some very abstract Pascal; much an Algol: there is not a lot of Pascal idiosyncrazies left specifically to allow to convert the programs to whatever. The simplest, nowadays, is to convert the Pascal like code in raw C.
In kerTeX this is what is made by kertex_M: the matrix, the tools needed to obtain C code. The matrix tools are compiled... on the matrix, to obtain C code that can be compiled for whatever target. (Allowing cross-compilation).
If you want to see the thing, just use kerTeX and with the get_mk_install.sh (or rc) program, when asked if you want the program to remove the intermediary products (including the *.c files in this case), just answer : NO to keep them.
For the bootstrapping problem (tangling tangle), D. E. Knuth has written that the first version was done by hand: he mimicked by hand what tangle would do (this is why such a program has to be basic, the same way a compiler has to be able to bootstrap a more basic version of itself to use itself to compile more complex one). This can also be done with text tools (I have sketched in kertex_M/bin1/tangle/tangleboot.sh such a program; it is not used, and lacks change file support).
The conversion from Pascal to C was needed because Pascal was not standardized, Pascal compilers not ubiquitous etc. And D.E. Knuth has also tried several years ago to encourage the possibility to compile TeX and al. with a Pascal compiler, just to be sure his programs will not be unavailable because of some compiling nightmare. And it was a pain.
It is more simple to convert to C, since it is, on purpose, a very basic Pascal (bibtex is far more hard to convert because it uses Pascal idiosyncrazies).
For information, the conversion from Web to C was web-to-c, initially from Tomas Rokicki. This is still what is used with TeXLive, or what has been used as a basis for kerTeX.