Aha! Not Sir William Jones after all, but rather Lord Monboddo, his correspondent, who suggested the following in a letter of 1789.
[I]f you can discover that central country from which all those nations, which you have named, have derived their affinity in language, manners and arts, which you observe, it will be a most wonderful discovery in the history of man. Of the three things I have named, by which the connection and relation of one nation to another is discovered, I hold Language to be the principal. […] And as it is the first of arts, so it is the most lasting, and one that travels the farthest, and is propagated to the most distant regions.
From Cannon (1968) in Am. Anth. [link]
Do read the wiki on Monboddo; he sounds an entertaining and erudite sort of thinker, and one of those minor Enlightenment figures who no doubt had more of an influence on the development of evolutionary thought than the standard textbooks give a hint of.