Edit
rsplus was removed from the table associated with the OP on April 14 with a commit note: not supported by this installation.
Not sure what this means, but Geshi has supported R in the past, although I note that he is not listed on the front page of the Geshi website. It was added to Geshi in version 1.0.8.5 (as indicated in the CHANGES file (fragment of it):
Version 1.0.8.5 - Added language files * AutoHotkey (Naveen Garg) * Awk (George Pollard) * GADV 4CS (Jason Curl) * jQuery (Rob Loach) * PowerBuilder (Doug Porter) * PureBasic (Gustavo Julio Fiorenza) * R / S+ (Ron Fredericks, Benilton Carvalho) ....
And there is no indication that it was deleted later.
So it looks like Geshi itself supports R, but maybe the Mediawiki plugin doesn't exist yet?
Original
You have completely read the list, haven't you ?; -)
It is listed in the second column under the name rsplus .
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