Interpretation and Translation

The first step of translation is interpreting the original source. The Book of Mormon refers to "interpreters," an apt name for the devices. 

Once the original source is interpreted, then translator does the second step of rendering the interpretation in the target language.

A good example of how this is done is the Rosetta stone.

Here, for example, is a word-for-word interpretation, followed by the translation into understandable English and German.

http://rosettastone.hieroglyphic-texts.net/sections/section-58/

(click to enlarge)

Joseph said the plates were written in "reformed Egyptian" and consisted of hieroglyphics. We can assume that the Urim and Thummim helped him interpret the characters engraved on the plates, but then he had to translate them using his own lexicon into English.