Dear Paolo,
Of the two Matsyapurana texts you mentioned, the first, Matsyapurana.
Ed. by J.D. Akhtar. Delhi: Oriental Publisher, 1972, is in fact a
photographic reprint of the English translation by "A Taluqdar of Oudh,"
published in 2 volumes in the Sacred Books of the Hindus series, 1916,
1917, as comparison will show, even though this is not stated. It does
not include the Sanskrit text. You can probably find the original Sacred
Books of the Hindus translation online.
While I do not have the second, Matsya Mahapurana (With an Exhaustive Introduction, Sanskrit Text, English Translation and Index of Verses).
Ed. by K.L. Joshi. New Delhi: Parimal Publication, 2007, the English
translation in this book may also be the one that was first published in
the Sacred Books of the Hindus. These publishers typically do not state
this. I have yet another reprint of the Matsyapurana, by Nag
Publishers, that also includes the English translation from the Sacred
Books of the Hindus, again unstated.
Best regards,
David Reigle
Colorado, U.S.A.