Congratulations on the package release and the JOSS article! It is absolutely a “labor of love”.
Since @Beliavsky has brought up other statistical software, it is worth mentioning that a number of the R stats base library procedures are in Fortran:
- Documentation: Help for package stats
- Source Files: r-source/src/library/stats at trunk · wch/r-source · GitHub
At one point I found a thread (it may have been a link or sub-page from the R Contributor site or the R Homepage; I couldn’t find it right now) about documenting the history of some of the FORTRAN codes and statistical algorithms used in R (or S?). As you may already know, the S statistical programming language from Bell Labs, was initially in FORTRAN (or RATFOR), until rewritten in C in 1988 (New S); more details in A Brief History of S (PDF, 151 KB).