Who is he that will plead with me? - That is, "who is there now that will take up the cause, and enter into an argument against me? I have set my cause before God. I appeal now to all to take up the argument against me, and have no fear if they do as to the result. I am confident of a sucessful issue, and await calmly the divine adjudication."
For now, if I hold my tongue I shall give up the ghost - This translation, in my view, by no means expresses the sense of the original, if indeed it is not exactly the reverse. According to this version, the meaning is, that if he did not go into a vindication of himself he would die. The Hebrew, however is, "for now I will be silent, and die." That is, "I have maintained my cause, I will say no more. If there is anyone who can successfully contend with me, and can prove that my course cannot be vindicated, then I have no more to say. I will be silent, and die. I will submit to my fate without further argument, and without a complaint. I have said all that needs to be said, and nothing would remain but to submit and die."

Who is he that will plead with me? - Let my accuser, the plaintiff, come forward; I will defend my cause against him.
I shall give up the ghost - I shall cease to breathe. Defending myself will be as respiration unto me; or, While he is stating his case, I will be so silent as scarcely to appear to breathe.

Who [is] he [that] will plead (h) with me? for now, if I hold my tongue, I shall (i) give up the ghost.
(h) To prove that God punishes me for my sins.
(i) If I do not defend my cause, every man will condemn me.

Who is he that will plead with me,.... Enter the lists with him; dispute the point, and try the strength of his arguments he had to plead for his own justification: thus Christ, the head of the church, and the surety of his people, is represented as speaking when he had by his obedience and sufferings made satisfaction for them, by bringing in an everlasting righteousness, and was, as their public and federal head, justified and acquitted, Isaiah 1:4; and much the same words are put into the mouth of a believer in him, and are expressed by him, Romans 8:33; who stands acquitted from all charges that men or devils, friends or foes, the law or justice of God, the devil and his own unbelieving heart, at any time, can bring against him. Job, well knowing the uprightness of his heart and life, the justness of his cause depending between him and his friends, boldly challenges them to come forth, and try it with him; or rather he seems desirous that God himself would take the case in hand, and plead with him; he was ready to engage with him, and in the presence of his friends, and in their hearing; and doubted not of being acquitted before God, and at his bar; so satisfied was he of his own innocence as to the things charged upon him:
for now, if I hold my peace, I shall give up the ghost; his sense seems to be, that if he was not allowed to speak for himself, and plead his cause, and have a hearing of it out, he could not live, he could not contain himself, he must burst and die; nor could he live under such charges and calumnies, he must die under the weight and pressure of them; though some think that this not only expresses his eagerness and impatience to have his cause tried fairly before God, but contains in it an argument to hasten it, taken from the near approach of his death: "for now", in a little time, "I shall be silent" (w); be in the silent grave: "I shall expire"; or die; and then it will be too late; therefore if any will plead with me, let them do it immediately, or I shall be soon gone, and then it will be all over: or rather the sense is, I challenge anyone to reason the matter, and dispute the point with me; and I promise that, if the cause goes against me, "now will I be silent"; I will not say one word more in my vindication: "I will die"; or submit to any death, or any sort of punishment, that shall be pronounced upon me; I shall patiently endure it, and not complain of it, or object to the execution of it; so Sephorno.
(w) "nune enim silebo et expirabo", Cocceius; so Schmidt, Schultens.

if, &c.--Rather, "Then would I hold my tongue and give up the ghost"; that is, if any one can contend with me and prove me false, I have no more to say. "I will be silent and die." Like our "I would stake my life on it" [UMBREIT].

The ghost - My grief would break my heart, if I should not give it vent.