according to this recent post
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
but maybe there are more up to date sources for this.