Wikipedia states R to be partially written in R, and other parts in C and Fortran. Recently, @KjellJorner initiated a discussion how to engage Fortran from Python with examples like f2py. Perhaps in contrast to you @Beliavsky, or others, I do not possess any working knowledge about R and still ask – out of curiosity*: is there a similar bridge, while working within R, to reach out for Fortran? For me, FortranCallsR found on the same index mentioned by you looks just the other way around …
*) In part equally a speculation: fortran-lang.org might benefit from a directory-like compilation in tune of «if you happen to work in language example, you may reach (back) to Fortran e.g., with module / library f2example with typical limitations of x and y». This guide need not to be exhaustive.