[ yes, this is "primarily opinion based;" VTC if you must ]
In Literate Programming Knuth wrote:
... I chose PASCAL as the programming language ... ; it is not my favorite language for system programming, but it has become a “second language” for so many programmers that it provides an exceptionally effective medium of communication. ...
It seems the C might have the same reach that PASCAL once did; although C++ is probably "better" than C in so many ways for a program like TeX (but C might be a "second language" for most C++ programmers). ECMAScript is also widely available and well known, although it's not really intended for a program like TeX. With C#/.NET now more-or-less cross-platform, maybe that's everybody's "second favorite language"? Of course, Java would be the "no Microsoft!" alternative to C#.
What language today might fulfill the same goals for TeX as PASCAL did in the early 80s? This is (mostly) a TeX question because of some of its unique goals; without such "requirements," TeX might still be in SAIL or would be written in PL/1.