As I mentioned in another question, I have seen inconsistencies in the recounting of the poem, First They Came .... This led me to look it up and find out that the English poem comes from a collection of interviews. I understand that it is based on speeches, but was it ever delivered as a poem in German? Was that prior to its English publication?
I'm not sure whether you'd call it a poem, but Niemöller definitely delivered a version of "First They Came ..." as a speech in German, as early as January 1946, before its translation into English.
Associate Professor Harold Marcuse has made a detailed study of Martin Niemöller and the evolution of "First They Came ...", which you can read about in great length on this page. He describes how Niemöller first began thinking about this issue:
Shortly after the end of the war Niemöller became convinced that the German people had a collective responsibility (he often used the word Schuld, guilt) for the Nazi atrocities. In October 1945 Niemöller was the the prime mover behind the German Protestant Church's "Confession of Guilt" ("Stuttgarter Schuldbekenntnis")(see this quotation from Oct. 1945). In later speeches Niemöller claimed that a November 1945 visit to Dachau, where the crematorium was being kept as a memorial site, began that process of recognition. (I tell his story of that visit in detail in my book Legacies of Dachau, excerpted here: Niemöller's postwar Dachau anecdote.)
Marcuse goes on to conclude that "First They Came ..." was already beginning to evolve in Niemöller's mind in October and November 1945. There were several versions of it, varying according to his audience, but we have a definite version dating from 6 January 1946, when Niemöller delivered the following speech in German to representatives of the Confessing Church in Frankfurt:
Als Pastor Niemöller ins Konzentrationslager kam, schrieben wir 1937, als das Konzentrationslager aufgemacht wurde, da schrieben wir 1933, und die damals in die Konzentrationslager kamen, waren Kommunisten. Wer hat sich darum gekümmert? Wir haben es gewußt, es stand in den Zeitungen. Wer hat die Stimme erhoben, etwa die Bekennende Kirche? Wir haben gedacht: Kommunisten, diese Religionsgegner, diese Christenfeinde - "soll ich meines Bruders Hüter sein?" Dann hat man die Kranken, die sogenannten Unheilbaren beseitigt. - Ich erinnere mich eines Gespräches mit einem Menschen, der Anspruch darauf erhob, ein Christ zu sein. Er meinte: Vielleicht ist es ganz richtig, diese unheibaren Menschen kosten den Staat nur Geld, sie sind sich und den andern nur zur Last. Ist es nicht das Beste für alle Teile, wenn man sie aus der Mitte schafft? -- Dann erst ist es an die Kirche als solche herangekommen. Dann haben wir einen Ton geredet, bis er dann in der Öffentlichkeit wieder verstummt ist. Können wir sagen, wir sind nicht schuld? Die Judenverfolgung, die Art und Weise, wie wir die besetzten Länder behandelten, oder die Dinge in Griechenland, in Polen, in der Tschechoslowakei oder in Holland, die doch in der Zeitung gestanden haben. … Ich glaube, wir Bekennende-Kirche-Christen haben allen Anlass, zu sagen: Meine Schuld, meine Schuld! Wir können uns mit der Entschuldigung, es hätte mich ja den Kopf kosten können, hätte ich geredet, nicht herausreden.
Wir haben es vorgezogen, zu schweigen. Ohne Schuld sind wir gewiss nicht, und ich frage mich immer wieder, was wäre geworden, wenn im Jahre 1933 oder 1934 - es muss ja eine Möglichkeit gewesen sein - 14 000 evangelische Pfarrer und alle evangelischen Gemeinden, die es in Deutschland gab, die Wahrheit bis in den Tod verteidigt hätten? Wenn wir damals gesagt hätten, es ist nicht recht, wenn Hermann Göring 100 000 Kommunisten einfach in die Konzentrationslager steckt, um sie umkommen zu lassen. Ich kann mir denken, dass dann vielleicht 30 000 bis 40 000 evangelische Christen um einen Kopf kürzer gemacht worden wären, kann mir aber auch denken, dass wir dann 30-40 000 Millionen [sic] Menschen das Leben gerettet hätten, denn das kostet es uns jetzt.
Translation into English by Marcuse (emphasis mine):
When Pastor Niemöller was put in a concentration camp we wrote the year 1937; when the concentration camp was opened we wrote the year 1933, and the people who were put in the camps then were Communists. Who cared about them? We knew it, it was printed in the newspapers. Who raised their voice, maybe the Confessing Church? We thought: Communists, those opponents of religion, those enemies of Christians - "should I be my brother's keeper?" Then they got rid of the sick, the so-called incurables. - I remember a conversation I had with a person who claimed to be a Christian. He said: Perhaps it's right, these incurably sick people just cost the state money, they are just a burden to themselves and to others. Isn't it best for all concerned if they are taken out of the middle [of society]? -- Only then did the church as such take note. Then we started talking, until our voices were again silenced in public. Can we say, we aren't guilty/responsible? The persecution of the Jews, the way we treated the occupied countries, or the things in Greece, in Poland, in Czechoslovakia or in Holland, that were written in the newspapers. … I believe, we Confessing-Church-Christians have every reason to say: mea culpa, mea culpa! We can talk ourselves out of it with the excuse that it would have cost me my head if I had spoken out.
We preferred to keep silent. We are certainly not without guilt/fault, and I ask myself again and again, what would have happened, if in the year 1933 or 1934 - there must have been a possibility - 14,000 Protestant pastors and all Protestant communities in Germany had defended the truth until their deaths? If we had said back then, it is not right when Hermann Göring simply puts 100,000 Communists in the concentration camps, in order to let them die. I can imagine that perhaps 30,000 to 40,000 Protestant Christians would have had their heads cut off, but I can also imagine that we would have rescued 30-40,000 million [sic] people, because that is what it is costing us now.
The first published English version I've been able to find was a translation of this speech published by the New York Philosophical Society in 1946. You can read it in full here, along with a short analysis of how this translation differs from the more direct one done by Marcuse. It was withdrawn from publication on 5 Aug 1946 after rumours appeared that Niemöller had supported the Nazis in the 20s.
Niemöller gave another speech a few months later, on 3 July 1946, entitled "Der Weg ins Freie" or "The Path into Freedom", as part of a series organized by the Ministry of the Interior of Württemberg and Baden. You can read it here along with an English translation and some analysis by Marcuse. This speech is sometimes claimed to be the origin of the "First They Came ..." quotation, but those precise words do not in fact appear there. I haven't been able to track down when those exact words were first published, but in any case, the spirit of the poem is there in the speech quoted above, which was delivered in German before it was ever translated to English.