Henry-Louis de La Grange's original Mahler Volume 1 was translated into American English and published by Doubleday & Company, NY, in 1973. That version seems to be readily available on the used market at quite reasonable prices.
Volumes 2–4 were translated into British English and published by Oxford University Press in 1995, 1999, and 2008, respectively. It's my understanding that Mr. de La Grange has finished his revision of volume 1 in French, and that its publication in English is imminent. I'm surprised that this was denied by someone at Oxford—perhaps Mr. de La Grange has decided to use a different publisher.
James