Pieper answered that question for me in his comments in publication of "Lehre und Wehre" (Doctrine and Defense). It was in the August 1919 issue, page 380, where he answered the question:
"Why the fathers of the Missouri Synod used the German language almost exclusively in the church."The use of the German language allowed them to keep the erroneous doctrines largely associated the "English" Lutheran churches and other "English" church bodies to infiltrate the precious, pure Christian doctrine of their church body. And so their German use was purely for spiritual reasons, not worldly reasons. See my later post for the writings of Pieper on this subject.
But God answered my plea that I might understand more writings of the fathers of the Missouri Synod in this digital/computer age. With a scanner, OCR software, and translation capabilities available even in the 1990s, I could begin to unlock the vast German writings of these men. But probably the largest share of my efforts in this work has been directed towards Dr. Franz Pieper since he brought true Lutheran (that is Christian) doctrine to our age like no other.... Franz Pieper, the "Twentieth Century Luther"!