Regarde, dit l'un d'eux.
...is a sentence in this book I'm reading and its translation is "Look, says one of them." The L in l'un is I'm assuming le, but then it would be "Look, says the one of them? Why l'un?
When an article is missing in an English sentence, it must be added to the French translation. The definite article can be used to fill this void in three situations:
Almost anywhere one would use "the" in English (i.e. when referring to specific things).
Before the subject of a sentence to state general truths about it.
Before the direct object of a verb of appreciation (like aimer) to express like/dislike.
If any of the above is true, then use the definite article. Otherwise, use the indefinite or partitive, depending on whether or not the noun is countable.
I like wine, but I am drinking milk. — J'aime le vin, mais je bois du lait.
The l' in this example is a quirk of formal language in French. It doesn't have a specific meaning or translation in English.
There's a little more information on it here: https://forum.wordreference.com/threads/lun.365123/