Or just chop off the last character if it's a comma:
IF improvementString[improvementString.length - 1] = ',' THEN
improvementString = Left$(improvementString, improvementString.length - 1)
END IF
In the lists of Improvements and National Wonders on a nation's page, there's a comma dangling at the end. Like this:
It's trivial but it looks a little funny with the extra comma.