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.

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.