If you know only one of the Neo-Latin languages, then you must understand quite a percent of Latin. Italian would be the best out of the bunch, as it is the closest to Latin. Portuguese and Spanish would be the runner ups, followed by French and Romanian.
From what I heard, it isn't that hard to learn.
Uhmm... You mean Welsh? There are like six Gaelic languages left: Welsh, Scottish, Irish, the one from Cornwell, the one from the Isle of Man and the one from Brittany.