I intend to use ML for molecular dynamics simulations and want to know of packages available in Fortran. Would it be more efficient to use some other language instead for the long run?
Probably not Fortran as the main language. Machine learning is a branch of Artificial Intelligence and needs an AI program like Python or similar systems. You might have some Fortran subroutines.
This is nice if you are already familiar with fortran; then you just need to make som wrapper functions on the R side. Otherwise you can go full R and dive into the praised H2O machine learning package. Python is another good choice.