I understand this is a Fortran forum, but combining multiple languages always involves a lot of extra programing/debugging/documentation/etc effort. In my opinion, for the overall effort to pay off, the Fortran/C/C++ part must be significant enough. However, you said:
- Computational bottleneck in a relatively small part of the entire code
In these cases, I find it it is preferable to stick to one language. For instance, if you chose Python, you can intentionally design the code such that the time-critical functions are well isolated and can be jitted with Numba. You get very decent performance and you don’t have to worry about writing bindings, compiling them, etc.
If you still want to go for Python+Fortran, here is a recent example of mine.
HugoMVale/odrpack-python: Python bindings for the modernized version of odrpack95.