Here's an odd tidbit I came across. While reading The Origin and Authority of the Biblical Canon According to the Continental Reformers (The Journal of Theological Studies, April 1907). Howarth says,
It is an interesting fact that earlier in the same year in which this disputation took place , Luther published a small tract entitled Eine kurze Unterweisung, wie man beichten soil. As an appendix to this tract, he published a translation of the Prayer of Manasses with the heading Des Konygs Manasses gebeth tzu der beicht ser dienstlich, and in the body of the tract, after quoting Ps. xxv n, he goes on to say:
'. . . wie denn des menicklich weyter erinnerung ausz des konigs Manasses tzu Juda gebeth nemen mag. Welches gebeth, weil es ser wol tzu der beicht dient, mag es ein utslichs christlichs mensch vor seiner beicht sprechen.'
It is strange that among Luther's very earliest Bible translations should be this prayer, which has been excluded from the canon by Roman Catholics and Protestants alike, and, as I believe, on quite inadequate grounds.
I was unaware that Luther translated the Prayer of Manasses, so I did a search in Luther's Works. I found the following quote from him in Volume 1 (Lectures on Genesis), commenting on Genesis 4:13. These comments come from late in Luther's life, notice as well his positive citation from the book of James.
13. And Cain said to the Lord: My iniquity is too great to be forgiven.
Here Moses appears to have introduced a perplexing difficulty for the linguists and the rabbis, for they torture this passage in various ways. Lyra mentions the opinions of some who explain the words as an affirmation, namely, that Cain, in his despair, said that his sin was too great to be forgiven, just as we also translate it. Augustine, too, adheres to this opinion. “Cain,” says he, “you lie. For God’s mercy is greater than the misery of all sinners.”
But the rabbis explain this as a question and in a negative sense: “My iniquity is not too great to be forgiven, is it?” But if this is the true meaning, Cain not only did not acknowledge his sin but even excused it and reproached God for inflicting a greater punishment than he deserved. In like manner, the rabbis distort the meaning of Scripture almost everywhere.
The reason for their going astray is that they are indeed familiar with the language but have no knowledge of the subject matter; that is, they are not theologians. Therefore they are compelled to twaddle and to crucify both themselves and Scripture. How is it possible to judge correctly about things that are unknown? The main thought in this passage is that Cain is being accused by his conscience. But there is no one—including not only any wicked person but even the devil himself—who would be able to endure this judgment. Thus James, in chapter two, declares that even the devils tremble in the presence of God (v. 19). And Peter, in his second epistle, chapter two, says that not even the angels, although they are greater in power and might, are able to endure the judgment which the Lord will bring upon those who blaspheme (v. 11). Manasseh, in his prayer, expresses it thus: “All men tremble before His wrathful face.” [LW 1:296]
I think it's safe to say that the simple citation of text doesn't imply Luther believed it to be canonical. So if a Roman Catholic apologist argues a positive citation from Luther equals an acceptance as canonical Scripture, they haven't weighed all the facts. This happens to be one of the main methods of argumentation put forth by Roman Catholic apologist Gary Michuta, not only for Luther, but for many other voices in Church history. It appears that the paradigm "citation=canon" is flawed.