How about turning them into a native date-time class, then re-formatting it.
For example, say you have some American dates in a "character" vector:
American.datechar <- c("5/15/1976","2/15/1970","1/9/2006")
# parse this:
American.date <- strptime(American.datechar,"%m/%d/%Y")
# reformat:
format(American.date,"%d/%m/%Y")