[drwells]

I have not done a census of the various libraries around, but all of the free finite element libraries in common use (deal.II, libmesh, and FEnICs come to mind first) are written in C++. I suspect Fluent and Abaqus are written in C (maybe Fortran?) but they are a bit older.

Trilinos is another C++ example, while PETSc and HYPRE are written in C.

My tentative conclusion from this is that newer projects tend to use C++ and slightly older projects use C.

Yes, there are things like ODEPACK, QUADPACK, and FFTPACK, but those are not under development anymore (as far as I know). The only widely used Fortran library still under development I can think of is LAPACK.

I did not count the occasional 'bespoke' code. There are still some Fortran applications in development for particular purposes (like MOM), but those are harder to survey.

[yiyus]

I do not think free finite element libraries are representative of commercial software. They usually include a mix of different languages, and that includes lots of Fortran, which is used both in the core (in solvers and linear algebra libraries, for example) and at the highest level for user subroutines. I have seen people using either Fortran or C++ with Abaqus, LS-Dyna or MSC, but nobody writting C (except for some very experimental solvers).

Most users are usually fine with whatever the GUI allows and a bit of Python, but for those of us developing new models, Fortran is probably the most useful language, followed by C++.