Any of the other Romance languages, the ones that evolved from Latin. The big ones are Portuguese, Romanian, Italian, and French, but there are many other minor ones: Catalan, Occitan, Romansh, Ladino, Sicilian, etc.
All Romance languages have a huge portion of vocabulary from Latin, and more similarities than differences in grammar when compared to non-Romance languages.
Also, zirp is right. Generally speaking, with every foreign language you learn, you get better at learning the next one, whatever that language is.