A lot of these translations were from the 20th century, with "thous" and "let thy"s found only in translated biblical verses and er, these translations of ancient works.
If I'm guessing, they're likely trying to convey the formality or informality of the pronoun used in the original, what's called the T - V distinction (from Latin, the informal tu vs the formal vos)
In English, thou was the informal, you the formal, as the subject in a sentence, with thee and ye the object form. Hence "God rest ye merry gentlemen" is better translated these days as "God rest you, merry gentlemen".
The T-V distinction is also why the Quakers maintained the thou form for some time, to indicate equality among all men (in the gender-neutral sense of the world), as you implied a difference in status.
The question of why the Bible used thous and thees is another interesting rabbit hole.
