Why are archaic English words often used in translations?

I often see the words "thou" and "thy" used in English translations from other languages, as opposed to the modern "you" and "your".

I thought this was something specific to translations of Sanskrit (when trying to create a historical or formal "mood"), but I recently saw the same thing with some other languages, e.g. a translation of the Polish national anthem.

Solution 1:

First, just to note—"thou" and "thy" aren't Old English. Old English, capitalized, generally refers to the language Beowulf was written in:

Hwæt. We Gardena in geardagum,
þeodcyninga, þrym gefrunon,
hu ða æþelingas ellen fremedon.

That said, the use of archaic words like "thou" is generally just an issue of style. Sometimes (such as when translating religious texts) it's meant to emulate the aesthetic of the King James Bible, which was deliberately archaic even for its time; other times it's just meant to sound formal and dignified.

Solution 2:

Sometimes, the words "thou", "thee", "thy" etc. are used in translations from other languages to disambiguate the singular and the plural. This has traditionally been especially important in bible translations, so "thee" and "thou" has come to be associated with those.

For example, Exodus 20:13 reads "Thou shalt not murder". It is translated from Hebrew, "לֹ֥֖א תִּֿרְצָֽ֖ח׃", that is, "don't kill". The source sentence was in the singular, that's why the translator chose "thou" over "you".