Greetings,
according to this recent post
https://forum.posit.co/t/r-rstudio-the-interoperability-environment-for-data-analytics/80083
more than 30% of the R codebase is written in Fortran. Moreover, contrarily to what some seem to ignore, the Fortran language standard is continuously evolving and offers a mature and convenient programming language for HPC to support R.
For these reasons, I think RStudio would be a much better piece of software if it supported Fortran syntax highlighting. A good starting point could be taking the keywords and schemes from here
https://www.winedt.org/old/Config/modes/Fortran90.php
but maybe there are more up to date sources for this.
Kind regards,
tiMo