Fortran, for some good and some not-so-good reasons. For heavy math crunching, a good reason is there are extensive libraries (BLAS, LAPACK) of tried-and-true subroutines, all written in Fortran (though those can be called from C and C++).

A not-so-good reason is the supposed performance advantage of Fortran over C/C++. Optimizers are pretty good, and few people understand that the benefit of optimizing a piece of code is proportional to the percent of time it is busy, which in almost all code is almost zero.

Another not-so-good reason is a culture gap between CS and non-CS programmers.
Scientific programmers tend to be taught bad habits in Fortran, and to look down on the CS programmers and the bad habits *they* have been taught, and who look down on the former.