The section titled המתרגם was authored by R. Mordekhai Plungian (מרדכי פלונגיאן). He was a Lithuanian maskil (i.e. active in promoting Haskalah attitudes/methods) and served as a proofreader on the Romm Talmud (Vilna) at which time he had the opportunity to contribute to its formation.
His name is not mentioned in any of the volumes of the Romm Shas, except for the very last volume all the way at the end in a section titled אחרית דבר. This epilogue contains the thanks and gratitude that the Romm publishers had for all those that contributed and helped in the production of their printing of Shas. In a small paragraph, in a smaller font than the rest of the page the following appears:
[This greatly magnified scan is from the NIL, if one tries to view it on HebrewBooks the print is so small as to make it illegible in the scan (bottom left side of the page, second to last paragraph)]
With great adulatory honorifics prefacing his name, one Rabbi Mordekhai ben Shelomo is identified for his contribution. Notably without his last name Plungian appended, somewhat cloaking his identity but leaving it to those in the know to clearly identify the author of המתרגם.
He also anonymously authored other works such as the מילואים in which he defends his ancestor R. Mordekhai Yoffe's Levush against the criticisms of the Shakh. The Steipler Gaon reported (קריינא דאגרתא, אגרת רנג, דף רסד) that R. Shemaryahu Yosef Karelitz (father of the Hazon Ish) instructed that the מילואים ought be treated suspiciously as the author's maskilic tendencies are well known to those that recognize him:
אולי ימצא איזו דבר שלא כהוגן בספר מלואים לא יקשה בעיני הקורא כי ידוע ומפורמם דרכו לכל יודעיו ומכיריו
It would then seem that R. Plungian published his composition quasi-anonymously in order so that it wouldn't deter traditionalists who were aware of his maskilic reputation, and in modern prints of the Talmud it continues to be published by Orthodox printing houses without naming its author altogether, in order to avoid enhancing the reputation of a maskil.