While the title page and the illustrations on Project Gutenberg match the 1888 Routledge edition, the text does not. Searching on Google books, one can find a scanned version of the Count of Monte Cristo that has the same text as the Project Gutenberg edition. This edition has copyright 1894, by Thomas Y. Crowell & Co. (the publishers). It includes a "Publisher's Note" (not included on PG) that gives some detail of the translation process. Unfortunately, it doesn't give the name of the translator.
Excerpted from the "Publisher's Note":
In the present revised edition, upwards of fifteen thousand such faults have been corrected. Great pains have been taken to represent the original faithfully in correct English. Technical terms have been expressed in accurate correspondents. The nautical phrases, so ludicrous in all other English translations, have been revised by a French Man-of-War's man, an Ex-Lieutenant of the United States Navy, and an experienced commander in the Merchant Service.
So an unnamed committee translated it, presumably based heavily on some earlier translation or translations. Wikipedia says that this translation was first published in 1889, and is based on an anonymous 1846 translation published by Chapman and Hall.