While I like various points from a number of highly upvoted answers here, I do not feel any one of them captures the whole picture, so I'll offer a compilation of what I see as the primary points to answer the question itself, some of which will obviously overlap certain of these other answers.
Are these mistranslations of the original?
It is not a mistranslation. The form of the Hebrew נָשׁ֥וּבָה is plural, evidenced by the plural prefix/form on the word; so "we will come back" is a proper translation.
If not, is it that Abraham knows that his son won't really die?
From the Genesis account, it is not revealed exactly what Abraham is thinking, other than in Gen 22:7c-8a he declares (ESV):
... He [Isaac] said, “Behold, the fire and the wood, but where is the lamb for a burnt offering?” 8 Abraham said, “God will provide for himself the lamb for a burnt offering, my son.” ...
This declaration relates to the following question from the OP as well:
Or is it only that he wishes to hide from his servants that he will kill his son?
And by extension, in v.8, is Abraham hiding from Isaac the plan? So with Abraham's declaration to his servants in v.5, and then more specifically his direct reply to Isaac's question in v.8, one of these four possiblities must be intended for what Abraham is thinking:
- Abraham is lying to both: Abraham is expecting to sacrifice his son and is lying to both the two young men and his son about who will be returning and what God has provided for.
- Abraham is telling the truth to both, version A: Abraham is expecting (trusting by faith), but not sure how, that God is going to provide a substitute sacrifice of the herd (שֶׂה can be translated sheep or goat, essentially an animal from a flock) for his son Isaac, such that he and his son will be returning to the other two.
- Abraham is telling the truth to both, version B: Abraham is expecting (trusting by faith), but not sure how, that God is going to provide some way for Isaac to return from this experience to see the young men again, even though he is later disclosing to Isaac that you "my son" are the chosen one from the flock to be the burnt sacrifice; this could be the case, as the Hebrew "my son" (בְּנִ֑י) comes immediately after the declaration of "the lamb for a burnt offering" (הַשֶּׂ֛ה לְעֹלָ֖ה) and so could be construed as in apposition to it (i.e. the lamb = you my son).
- Abraham is lying to the young men, but telling Isaac the truth: If the apposition noted in #3 is correct for v.8, and so telling the truth to Isaac, but Abraham is not really expecting Isaac to return from this, then he was lying to the two young men.
Now from the Genesis 22 passage itself, the incident itself ends up matching to #2 (v.13). Yet at the same time, before that substitution was made, Abraham was following through with the request of God to offer Isaac as the burnt offering (v.2) as he took up the knife with his son on the altar (v.10), which matches the #1, 3, 4 of actually intending to sacrifice.
To answer from the context, #1 is more likely than #4. If Abraham is going to lie, he might as well go all out. But Abraham had learned some lessons about how lying (or at least giving a half-truth, Gen 20:12) can cause great harm (Gen 20:9) because of lack of faith (Gen 20:11). He had established himself as a man of integrity after that (Gen 21:22-33). So lying, at this point of testing of his faith, does not appear to the idea being relayed in the context here in Gen 22:5 or 8.
If that logic is accepted, then neither #1 or #4 appear to be the best possiblity, and so the answer to the third question is "No," he is not lying to the servants.
Possibilities #2 & 3, however, point out that Abraham does not really "know" for sure "that his son won't really die," but only that by faith, he is "trusting" that God has something planned that He has not yet revealed. This trusting disposition seems strongly implied both:
- by God's declaration to Abraham when he stops him (v.12), as God sees Abraham's heart as willing to sacrifice Isaac for God:
He [YHWH] said, “Do not lay your hand on the boy or do anything to him, for now I know that you fear God, seeing you have not withheld your son, your only son, from me.”
- by Abraham's declaration that God followed through to provide (v.14):
So Abraham called the name of that place, “The LORD will provide”; as it is said to this day, “On the mount of the LORD it shall be provided.
Evidence from the passage suggests that Abraham was trusting God, expecting in some way that God was going to have Isaac return alive with him to his servants—whether by substitution of the sacrifice required, which did occur, or by some means of following through with the sacrifice.
As to the latter, if one holds to the New Testament commentary on that as being divinely inspired (as I do), then Hebrews 11:17-19 (as this answer, though incomplete in answering the OP's question because of jumping straight to the New Testament) expresses well that perspective of what Abraham was thinking if he did have to go through with sacrificing his son.
So I believe Abraham, at different times while pondering God's request on the journey to fulfill it, was thinking of at least the two possible solutions of #2 (substitute sacrifice) or #3 (resurrection from death) for God to keep His promise to him of Isaac's role (Gen 17:19).
But even if one does not want to consider #3 as valid (by rejecting the New Testament commentary on that), then #2 still is the better answer in the context of the book of Genesis than either #1 or #4.
In either case, the answer as to the main question of
Why did Abraham say “We shall return” before sacrificing Isaac?
appears to be that he, by faith, was expecting Isaac to in some way return, despite God's call for his son's sacrifice.