You can find an English translation in the following article: Joranson, Einar. “The Problem of the Spurious Letter of Emperor Alexius to the Court of Flanders.” The American Historical Review 55, no. 4 (1950): 811–32.

In general, the letter has been taken to be a fake designed to rally support in the early twelfth century against Byzantium. The scholarship here is mostly in German and I haven't personally read it so I won't refer to it, but recently the idea that it was based upon a real letter that Alexios sent has been proposed: Frankopan, Peter. The First Crusade: The Call from the East. Cambridge, MA: Harvard University Press, 2012, 60.