It's not for the sake of veracity
Whether the historical Caesar pronounced or not the said words is disputed. Of five Antique sources on Caesar's death:
- Nicolaus of Damascus, Plutarch and Appian do not report the quote
- Suetonius and later Cassius Dio report it only as a dubious variant to the tradition, but using Ancient Greek rather than Latin: Και συ τέκνον
There is debate as to why Caesar would have switched to Greek for this admonition but as every Roman in the upper circles, Caesar was probably raised in a Greek-speaking environment before learning Latin, and this can be interpreted as a regression to the language of his childhood.
The fact that no ancient sources report the quote in Latin has lead the French and Italian versions to retain another translation, Tu quoque mi fili, probably coined by Lhomond in the 18th century, which brings us to why Shakespeare used another translation of the quote to Latin, Et tu, Brute, in his masterpiece.
It sounded OK to the medieval ear - or even stylish
Mixing Latin and vernacular language was a widespread practice in Medieval Europe. Latin was still the language of scholars and clerics, while vernacular language was taking the spot gradually, so mixing both languages would not have shocked an educated audience in the 14th century. But Shakespeare worked later, and his inclusion of a Latin hemistich was probably motivated by style consideration, mimicking a practice that came to life in 15th century Italy where poets would alternate verses in Italian and Latin. It was probably considered fashionable by Shakespeare to include some Latin at climactic moments of his plays, you can find a list in the article Latin on ShakespearesWords.com.
The sentence was probably proverbial in English at the time
Shakespeare himself used it (in Latin) in Henry VI and Richard Edes features it in Caesar Interfectus in 1582 (17 years before the play we're discussing), which hints toward the fact that the sentence was probably often quoted in the context of similar fates for those that reach power before being used in the play. The audience, I would say, would have been expecting the line and waiting for it rather than shocked by its appearance.
Being "true to life" is by no means one of the objectives of Shakespeare's plays, or at least not being constantly true to life. Alternating between ghosts and lively maid scenes was the man's genius, and realism should definitely not be judged in this case using modern standards.