The earliest reference I can track down is from 1952. In Roger Sessions: a biography (2008), Andrea Olmstead writes:

[In 1952] Sessions published "Some notes on Schoenberg and the 'method of composing with twelve tones'." At the head of the article he quoted from one of Schoenberg's letters to him: "A Chinese philosopher speaks, of course, Chinese; the question is, what does he say?" Sessions [had performed] the role of a Chinese philosopher in Cleveland.

(The work that Sessions had performed this role in appears to have been Man who ate the popermack in the mid-1920s.)

Sessions' essay (originally published in The Score and then collected in Roger Sessions on Music) begins:

Arnold Schönberg sometimes said 'A Chinese philosopher speaks, of course, Chinese; the question is, what does he say?' The application of this to Schönberg's music is quite clear. The notoriety which has, for decades, surrounded what he persisted in calling his 'method of composing with twelve tones', has not only obscured his real significance, but, by focusing attention on the means rather than on the music itself, has often seemed a barrier impeding a direct approach to the latter.

An entertaining later reference to this quotation appears in Dialogues and a diary by Igor Stravinsky and Robert Craft (1963), where Stravinsky tabulates the differences between himself and Schoenberg, culminating in this comparison:

Stravinsky: ‘What the Chinese philosopher says cannot be separated from the fact that he says it in Chinese.’ (Preoccupation with manner and style.)

Schoenberg: ‘A Chinese philosopher speaks Chinese, but what does he say?’ (‘What is style?’)