" The tendency to ignore the distinction between adjectives and adverbs doubtless results from the influence on British speakers and writers of American speech, in turn strongly influenced by German, which hardly recognises a difference between adjectives and adverbs."
The author of those words is Michael Dummett, who was "Wykeham Professor of Logic at Oxford."
In his Grammar and Style for examination candidates and others (1993), he also says: "Phrases like to act natural [his emphasis] and he doesn't see good are pure Germanisms, and, though in common use by Americans, are recognised as slang."