For now, I named it language-change. language-history feels better. But I'm almost sure there's established term for this, which I don't know. Should I change the tag name? It's early enough to do this easily.
The term linguists use, which may be the official one you're looking for, is "diachronic change." That may be a little jargony for non-linguist users, though. (It occurs as part of the field of study known as "diachronic linguistics" or "historical linguistics." But I think those would be worse, as well as not in fact accurate.)