Paul was born in Tarsus and probably spoke Greek since his childhood. He was most likely born a Roman citizen for being born in Tarsus. It's not surprising he is talking to the Greeks this way. He is trying to convert them, and this is what he has to do.
It is true Paul was a Roman citizen, which he did not avail much during the ministry except maybe after the fact... ( one would think Paul could have led with that to prevent unnecessary afflictions but he did not. )
Acts 16:36 And the keeper of the prison told this saying to Paul, The magistrates have sent to let you go: now therefore depart, and go in peace.
37 But Paul said unto them, They have beaten us openly uncondemned, being Romans, and have cast us into prison; and now do they thrust us out privily? nay verily; but let them come themselves and fetch us out.
38 And the serjeants told these words unto the magistrates: and they feared, when they heard that they were Romans.
,....but he was a full fledged Pharisee.
Philippians 3:1Finally, my brethren, rejoice in the Lord. To write the same things to you, to me indeed is not grievous, but for you it is safe.
2 Beware of dogs, beware of evil workers, beware of the concision.
3 For we are the circumcision, which worship God in the spirit, and rejoice in Christ Jesus, and have no confidence in the flesh.
4 Though I might also have confidence in the flesh. If any other man thinketh that he hath whereof he might trust in the flesh, I more:
5 Circumcised the eighth day, of the stock of Israel, of the tribe of Benjamin, an Hebrew of the Hebrews; as touching the law,
a Pharisee;
6 Concerning zeal, persecuting the church; touching the righteousness which is in the law, blameless.
7 But what things were gain to me, those I counted loss for Christ.
8 Yea doubtless, and I count all things but loss for the excellency of the knowledge of Christ Jesus my Lord: for whom I have suffered the loss of all things, and do count them but dung, that I may win Christ,
9 And be found in him, not having mine own righteousness, which is of the law, but that which is through the faith of Christ, the righteousness which is of God by faith:
10 That I may know him, and the power of his resurrection, .....
Paul often contradicts Jesus Christ in the Gospels. Much of what Paul says directly contradicts Matthew 5:17-20. Neither earth or heaven has passed away yet, so the law still remains.
Jesus Fulfills the Law
17Think not that I am come to destroy the law, or the prophets: I am not come to destroy, but to fulfil. 18For verily I say unto you, Till heaven and earth pass, one jot or one tittle shall in no wise pass from the law, till all be fulfilled. 19Whosoever therefore shall break one of these least commandments, and shall teach men so, he shall be called the least in the kingdom of heaven: but whosoever shall do and teach them, the same shall be called great in the kingdom of heaven.
20For I say unto you, That except your righteousness shall exceed the righteousness of the scribes and Pharisees, ye shall in no case enter into the kingdom of heaven.
Paul did not contradict Jesus Christ at all.
Consider the very words of Jesus you had quoted and discern the meaning of verse 19. How can anyone break the least of His commandments and teaches others so, be called the least and yet still in His kingdom of Heaven? For anyone to have his or her righteousness to exceed the righteousness of the scribes and the Pharisees, how can they even break the least of His commandments and still be in the kingdom of heaven but as the least?
It is because they have His righteousness which is apart from the law, the righteousness of faith in Jesus Christ. Jesus fulfilled the law in such a way that as Jews under the Old Covenant in trying to keep the law to obtain salvation and to live as His, Jesus saved all those that believe in Him, thus having paid the ransom for sin which is eternal life in Jesus Christ our Lord.
Romans 3:26 To declare, I say, at this time his righteousness: that he might be just, and the justifier of him which believeth in Jesus.
27 Where is boasting then? It is excluded. By what law? of works? Nay: but by the law of faith.
28 Therefore we conclude that a man is justified by faith without the deeds of the law.
Colossians 1:20 And, having made peace through the blood of his cross, by him to reconcile all things unto himself; by him, I say, whether they be things in earth, or things in heaven.
21 And you, that were sometime alienated and enemies in your mind by wicked works, yet now hath he reconciled
22 In the body of his flesh through death, to present you holy and unblameable and unreproveable in his sight:
23 If ye continue in the faith grounded and settled, and be not moved away from the hope of the gospel, which ye have heard, and which was preached to every creature which is under heaven; whereof I Paul am made a minister;
So when you read Matthew 5:17-20, think of how Jesus words would apply there in that no matter how much the spirit is willing, the flesh is weak, therefore what is impossible with man, is possible with God by believing in Jesus Christ.
No Christian is under the law to keep the sabbath day holy because they are saved. The law of the sabbath has not been done away with, but Jesus has saved them and dwells in them as their bodies are the temples of the Holy Spirit for why they are guiltless for profaning the sabbath day because Jesus has saved them.
Matthew 12:1At that time Jesus went on the sabbath day through the corn; and his disciples were an hungred, and began to pluck the ears of corn and to eat.
2 But when the Pharisees saw it, they said unto him, Behold, thy disciples do that which is not lawful to do upon the sabbath day.
3 But he said unto them, Have ye not read what David did, when he was an hungred, and they that were with him;
4 How he entered into the house of God, and did eat the shewbread, which was not lawful for him to eat, neither for them which were with him, but only for the priests?
5 Or have ye not read in the law, how that on the sabbath days the priests in the temple profane the sabbath, and are blameless?
6 But I say unto you, That in this place is one greater than the temple.
7 But if ye had known what this meaneth, I will have mercy, and not sacrifice, ye would not have condemned the guiltless.
8 For the Son of man is Lord even of the sabbath day.
1 Corinthians 6:19 What? know ye not that your body is the temple of the Holy Ghost which is in you, which ye have of God, and ye are not your own?
20 For ye are bought with a price: therefore glorify God in your body, and in your spirit, which are God's.
2 Corinthians 13:5 Examine yourselves, whether ye be in the faith; prove your own selves. Know ye not your own selves, how that Jesus Christ is in you, except ye be reprobates?
Since Jesus gave 2 examples of Old Testament saints profaning the sabbath but were guiltless because they were in the Temple, so are His disciples and all saved believers are guiltless for profaning the sabbath because He has saved them & is with them always for why His righteousness in us has exceeded the righteousness of the scribes & the Pharisees, and that righteousness of God is of faith in Jesus Christ of what the law failed to do for us, God has done it. Period.
Anyway, saved believers not found abiding in Him as His disciples being a bigot will be left behind but they are still saved which is why they shall be called least in the kingdom of heaven for breaking the least of his commandments and teaching others so thus being "damned" as vessels unto dishonor in His House BUT they are still in His House testifying to the power of God in salvation for all those that believe in Him, even in His name..
2 Timothy 2:19 Nevertheless the foundation of God standeth sure, having this seal, The Lord knoweth them that are his. And, let every one that nameth the name of Christ depart from iniquity.
20 But in a great house there are not only vessels of gold and of silver, but also of wood and of earth; and some to honour, and some to dishonour.
21 If a man therefore purge himself from these, he shall be a vessel unto honour, sanctified, and meet for the master's use, and prepared unto every good work.
Better believe Jesus will finish His work in them that are disqualified to attend the Marriage Supper in Heaven as they will be scourged till they become partakers of His holiness by being a bigot no more.