In the February 1960 and May 1965 printings of the 1952 Alfred A. Knopf edition, the March 1969 Vintage Books edition, and the 1992 Modern Library edition, there is an author's note at the back of the book called "The Making of The Magic Mountain," which was originally printed in the January 1953 issue of The Atlantic Monthly.
That author's note contains the following passage:
Oddly enough, it is not a difficulty for me, but rather the reverse, that I have to discuss The Magic Mountain in English. I am reminded of the hero of my novel, the young engineer Hans Castorp. At the end of the first volume, he makes an extraordinary declaration of love to Madame Chauchat, the Kirghiz-eyed heroine, veiling its strangeness in the garment of a foreign tongue. It eases his embarrassment and helps him to say things he could never have dared to say in his own language. "Parler français," he says, "c'est parler sans parler, en quelque manière." In short, it helps him over his inhibitions—and an author who feels embarrassed at having to talk about his own works is in the same way relieved at being able to talk about them in another language.
The Atlantic website has archives of some past issues, but while it has November 1952 and March 1953, it is missing the January 1953 issue, and I do not have access to issues that old, so I am unable to confirm whether this essay appeared in the January 1953 issue of The Atlantic Monthly. I do not have access to an old Readers Guide to Periodical Literature, which would index the original article, and none of the online indexes I can find list this article.
However, you can read the full author's note in any of the editions of The Magic Mountain linked in the first paragraph of this answer.