David's hair, Absalom's hair, and why translations keep some words

• Why do English Bible translations sometimes leave words like raca untranslated?

A comedy detour into whether David had curly, 'luscious' hair and the son whose thick hair caught in an oak tree leads to a real question: why do English translations sometimes break back to the original words, like 'raca' or Christ's words on the cross? The answer offered: transliteration happens when translators cannot agree on a rendering -- examples given include baptism, angel, and Sanhedrin.