Many languages have terms for siblings that define whether one is younger or older in relation to another sibling.
The Greek that is translated as “a man’s brother” in English is translated in Chilcotin as ˀeyen ya ˀeŝqi ˀatalilh gwech´ez bunagh (“a younger brother who was born after [a man] his older brother”). (Source: Quindel King)
Many languages distinguish between inclusive and exclusive first-person plural pronouns (“we”). The inclusive “we” specifically includes the addressee (“you and I and possibly others”), while the exclusive “we” specifically excludes the addressee (“he/she/they and I, but not you”). This grammatical distinction is called “clusivity.” While Semitic languages such as Hebrew or most Indo-European languages such as Greek or English do not make that distinction, translators of languages with that distinction have to make a choice every time they encounter “we” or a form thereof (in English: “we,” “our,” or “us”).
For this verse, translators typically select the inclusive form (including Jesus).
Source: Velma Pickett and Florence Cowan in Notes on Translation January 1962, p. 1ff.
Following are a number of back-translations of Mark 12:19:
Uma: “‘Teacher, the prophet Musa wrote in the book of the law like this: if a man dies yet has no children, his brother must marry his widow so that he will have descendants.” (Source: Uma Back Translation)
Yakan: “‘Sir, there is a law that Musa has given to us (incl.) that if there is a man and he dies without having children, (then) his younger-brother should marry that widow in order that there will be descendants of his older-brother.” (Source: Yakan Back Translation)
Western Bukidnon Manobo: “And they said to Jesus, they said, ‘Teacher, Moses taught long ago that, for example, if there is a man who has a wife and that man dies, it is necessary for the brother of that man to marry his sister-in-law so that the dead person might have children through him.” (Source: Western Bukidnon Manobo Back Translation)
Kankanaey: “‘Sir teacher, our law which Moses wrote says, ‘If there is a married-couple who have no children and the man dies, his brother must marry the widow so that thus if they have a child, that will be like a child of the dead-one.'” (Source: Kankanaey Back Translation)
Tagbanwa: “‘Teacher,’ they said, ‘we (incl.) Judio have a law which was written by Moises, that if a man who is married dies, that married couple having no children yet, he is to be succeeded by his brother in marrying (that wife). For if they have a child, it is to be regarded as like it is indeed the child of that dead (person).” (Source: Tagbanwa Back Translation)
The name that is transliterated as “Moses” in English is signed in Spanish Sign Language in accordance with the depiction of Moses in the famous statue by Michelangelo (see here). (Source: John Elwode in The Bible Translator 2008, p. 78ff.)
Another depiction in Spanish Sign Language (source: Carlos Moreno Sastre):
The horns that are visible in Michelangelo’s statue are based on a passage in the Latin Vulgate translation (and many Catholic Bible translations that were translated through the 1950ies with that version as the source text). Jerome, the translator, had worked from a Hebrew text without the niqquds, the diacritical marks that signify the vowels in Hebrew and had interpreted the term קרו (k-r-n) in Exodus 34:29 as קֶ֫רֶן — keren “horned,” rather than קָרַו — karan “radiance” (describing the radiance of Moses’ head as he descends from Mount Sinai).
Even at the time of his translation, Jerome likely was not the only one making that decision as this recent article alludes to.