Why do titles of Mary end with the "-trix" suffix (as in Mediatrix, Genetrix, Coredemptrix)?
The answer to this question is quite simple. In Latin, the masculine ending for an agent is "-tor" and the feminine ending for an agent is "-trix."
For example, Christ is the SalvaTOR ("Savior") and RedempTOR ("Redeemer") of humanity. A male merchant is a "mercaTOR," and a male governor is a "gubernaTOR." However, a female merchant would be a "mercaTRIX," and a female governor would be called a "gubernaTRIX."
Incidentially, Queen Elizabeth II of England still bears the official title of "Gubernatrix Suprema" of the Church of England.
You can still hear the Indo-European likeness in English nouns denoting agency. For example, a "waiTER" is male and a "waiTRESS" is female. The masculine English ending "-ter" corresponds with the Latin masculine ending "-tor," and the English feminine ending "-tress" corresponds to the feminine Latin ending "-trix."
So the Marian title "Mediatrix" could be translated into English as "Mediatress." However, one usually sees it in English literature with the authentic Latin ending: Mediatrix.