Arianell fails what I personally call the "Facebook test": Among the 400 million or so current members of Facebook there seems to be not a single person with that name. In my opinion, this indicates a low probability that Arianell is *currently* in any significant use anywhere, be it in Wales or in other places.
Searches for Arianelle (with an "e" added) and Arianella (with an "a" added) turn up only a handful of people on Facebook that might come from countries with Romance languages, according to their family names.
Rene
www.AboutNames.ch