It does not say "mankind." That was added in translation which leads many astray to think and believe mankind is actually in view when Joel prophesied against Israel.
Look at the text. The text itself tells you WHO the prophet was addressing. He was addressing Israel and the context is Israel: Joel is speaking to Israel, the southern kingdom of Judah to be precise.
27 And ye shall know that I am in the midst of Israel,
And that I am the LORD your God, and none else:
And my people shall never be ashamed.
28 And it shall come to pass afterward,
That I will pour out my spirit upon all flesh; (MEANING ALL HEBREW/JEWISH FLESH.)
And your sons and your daughters shall prophesy,
Your old men shall dream dreams,
Your young men shall see visions:
29 And also upon the servants and upon the handmaids
In those days will I pour out my spirit.
Joel 2:27–29.
The "your" refers to Israel. The Spirit was Promised to Israel. And when God fulfilled this Promise by sending the Holy Spirit of Promise spoken of by Peter, three thousand Jews were born-again and saved in one day.
1 And when the day of Pentecost was fully come, they were all with one accord in one place.
2 And suddenly there came a sound from heaven as of a rushing mighty wind, and it filled all the house where they were sitting.
3 And there appeared unto them cloven tongues like as of fire, and it sat upon each of them.
4 And they were all filled with the Holy Ghost, and began to speak with other tongues, as the Spirit gave them utterance.
Acts 2:1–4.
The birthday of the Jewish Church began on the day of the Jewish Feast of Harvest (ca. 34 AD.) On that morning eleven Jewish men were filled with the Holy Spirit of Promise PROMISED TO ISRAEL.)
God promised His Spirit to His covenant people Israel.
Below, Peter is speaking to a Jewish crowd that gathered outside the upper room.
38 Then Peter said unto them, Repent, and be baptized every one of you in the name of Jesus Christ for the remission of sins, and ye shall receive the gift of the Holy Ghost.
39 For the promise is unto you, and to your children, and to all that are afar off, even as many as the Lord our God shall call. Acts 2:38–39.
God does the calling, and He calls Jewish men and women. He does not call Gentiles for Gentiles are not under the Law nor are Gentiles the inheritors of God's promises. God never promised Gentiles anything. God is not obligated to do or give Gentiles anything. He has no covenant with Gentiles, and they have no spiritual insurance against sin and death. But Israel does. The covenants God made first with Abram the Hebrew extended it to his son (Isaac) and then to his grandson, Jacob. Jesus is a Jewish Messiah, NOT a Gentile messiah.
Looks like a repetition
Joel 2:28 (NASB95) — 28 “It will come about after this
That I will pour out My Spirit on all mankind; And your sons and daughters will prophesy, Your old men will dream dreams, Your young men will see visions.
Joel 2:28 (LEB) — 28 And it will happen afterward thus: I will pour out my Spirit on
all flesh, and your sons and your daughters will prophesy, and your elders will dream dreams; your young men shall see visions.
Joel 2:28 (KJV 1900) — 28 And it shall come to pass afterward, That I will pour out my spirit upon
all flesh; And your sons and your daughters shall prophesy, Your old men shall dream dreams, Your young men shall see visions:
Joel 2:28 (UASV) — 28 “And it will happen afterward, that I will pour out my Spirit on
all flesh; your sons and your daughters will prophesy, your old men will dream dreams, and your young men will see visions.
Joel 2:28 (NIV) — 28 “And afterward, I will pour out my Spirit on all people. Your sons and daughters will prophesy, your old men will dream dreams, your young men will see visions.
all people, all flesh mankind
The Spirit is to be poured out on all
And all nations are to see salvation
Isa 49:6He says, “It is too small a thing that You should be My Servant To raise up the tribes of Jacob and to restore the preserved ones of Israel;
I will also make You a light of the nations So that My salvation may reach to the end of the earth.”
that is a promise to nations unto the end of the earth.
Ps 67:2That Your way may be known on the earth, Your salvation among all nations.
A PROMISE OF SALVATION TO ALL NATIONS
And all nations are to be blessed
Genesis 18:18 (NASB 95) — 18 since Abraham will surely become a great and mighty nation, and
in him all the nations of the earth will be blessed?
A promise of blessings for all nations.
Gen 22:18 In your seed
all the nations of the earth shall be blessed, because you have obeyed My voice
As Abram is the father of all nations
”
Gen 17:5“ No longer shall your name be called Abram, But your name shall be Abraham;
For I will make you the father of a multitude of nations.
Then of course a even larger New testament blessing, indwelling and salvation for all.
John 3:16–17“For God so loved the world, that He gave His only begotten Son, that whoever believes in Him shall not perish, but have eternal life. For God did not send the Son into the world to judge the world, but that the world might be saved through Him.
Rom 10:9–13that if you confess with your mouth Jesus as Lord, and believe in your heart that God raised Him from the dead, you will be saved; for with the heart a person believes, resulting in righteousness, and with the mouth he confesses, resulting in salvation. For the Scripture says, “Whoever believes in Him will not be disappointed.” For there is no distinction between Jew and Greek; for the same Lord is Lord of all, abounding in riches for all who call on Him; for “Whoever will call on the name of the Lord will be saved.”
1 Tim 2:3–6This is good and acceptable in the sight of God our Savior, who desires all men to be saved and to come to the knowledge of the truth. For there is one God, and one mediator also between God and men, the man Christ Jesus, who gave Himself as a ransom for all, the testimony given at the proper time.
2 Pet 3:9The Lord is not slow about His promise, as some count slowness, but is patient toward you, not wishing for any to perish but for all to come to repentance.
Titus 2:11For the grace of God has appeared, bringing salvation to all men,
Rom 5:18So then as through one transgression there resulted condemnation to all men, even so through one act of righteousness there resulted justification of life to all men.
1 John 2:2and He Himself is the propitiation for our sins; and not for ours only, but also for those of the whole world.
Ezek 18:23Do I have any pleasure in the death of the wicked,” declares the Lord God, “rather than that he should turn from his ways and live?
John 1:29The next day he saw Jesus coming to him and said, “Behold, the Lamb of God who takes away the sin of the world!
1 Tim 4:10For it is for this we labor and strive, because we have fixed our hope on the living God, who is the Savior of all men, especially of believers.
Isa 45:22“Turn to Me and be saved, all the ends of the earth; For I am God, and there is no other.
Rom 3:23–24for all have sinned and fall short of the glory of God, being justified as a gift by His grace through the redemption which is in Christ Jesus
Gal 3:28There is neither Jew nor Greek, there is neither slave nor free man, there is neither male nor female; for you are all one in Christ Jesus.
Col 1:19–20For it was the Father’s good pleasure for all the fullness to dwell in Him, and through Him to reconcile all things to Himself, having made peace through the blood of His cross; through Him, I say, whether things on earth or things in heaven.
Heb 2:9But we do see Him who was made for a little while lower than the angels, namely, Jesus, because of the suffering of death crowned with glory and honor, so that by the grace of God He might taste death for everyone.