When, I ask myself will English Bible translators have the guts to transliterate ραββουνι in John 20:16; Mark 10:51 as Rabbuni rather than Rabboni? Obviously the reading ραββωνι is early and has come via the Latin tradition to find a place in many European Bible translations. You can find Rabboni or something quite like it in English, Spanish, Portuguese and Italian—even Albanian. However, unless textual critics are going to argue that the form with omega is original they surely ought to give us a more precise transliteration of the spelling ραββουνι.
What got into the ESV committee that they rendered the term as Rabbi in Mark 10:51 and Rabboni in John 20:16?