The Southwestern States Mission during the time of this study included significant populations of French-, German-, and Spanish-speaking citizens. A variety of other languages were also spoken. For the missionaries in this study (mostly in Texas), German was the most common non-English language encountered, followed by Polish. 
Elder Brooks had the most contact with German speakers, but almost all the missionaries recorded encounters with “Germans.”  In the following entry Elder Brooks touches on most of the elements of the Mormon / German interactions as recorded in the diaries:
We left that gentleman’s place and started on our way. The weather was clear and quite mild. What white people there were, were Germans. There were very few American people living in that part of the county. We went through the settlement distributing tracts. A great many of the Germans could not speak English. They seemed to be a very peculiar people. They would have but very little to do with us. Each family had from one to four dogs, and savage ones at that. They did not seem very particular about calling them back. When noon came we tried to talk them out of our dinner but we could not do it. We came on into a little town called William Penn. About the first thing we met was a Lutheran Minister who gave us a pretty stiff argument but we came out victorious. It was then nearly night. We inquired for shelter. Stopped over night with a doctor. He was a German. The minister we met was a German also. The doctor’s name was Knolely. 
In the missionaries’ racial and national taxonomies, people of North-European descent who did not speak English well or otherwise maintained an immigrant identity were “white” but not “American.” I don’t have enough data to assert a clear connection, but the diaries only report three violent (or nearly so) encounters with dogs, and all three occur in neighborhoods with Germans or Poles.  The food seemed to be good, though: “The German people are good hands making light bread.” 
German and other European settlers followed fairly typical cluster-migration patterns and some of the communities seem to have resisted cultural encroachment:
“We were informed that the other trustees of that school house were Germans. We thought it wasn’t worth while to try and see them as the people tell us they were greatly opposed to Americans holding meetings in the school house.” 
When working in those areas, the missionaries often reported difficulties with the language: “Stoped at night with a German. I can’t say his name”; “He was a German and could not understand us very good.”  The missionaries distributed tracts anyway, most of which seem to have been printed in English, but there is one instance of an explicitly German-language tract. 
The majority of the diary entries that mention European-descent focus on the challenge to find a place to sleep. For examples:
“There were a number of American families living close there but we had to stop with a German. It seemed as though we couldn’t stop with the American people.”
“There were very few white people living in that part of the country. What few there were, were mostly Polanders. …Night was coming on. We asked for shelter but the man we asked could not take us in. He said his wife was sick but it was more prejudice than any thing else. There were no more American people living closer than two miles from there. Our only show was to stop with a Polander and they were generally hard to stop with. We went and asked one to let us stay. We could hardly make him understand what we wanted but he finally took us in. They went and got a Polander that could talk English. We then made them understand what we wanted. They treated us fine. They had but one bedstead. They gave it to us and they slept on the floor. I could not find out what their names were.” 
The language barrier was reported in Texas, Oklahoma, and Kansas. In 1906 President Duffin “made request for a number of German speaking elders to labor in the Central States Mission as there are large numbers of Germans to whom our elders cannot present the gospel, not being able to speak the language.”  I do not know what came of the request.
The “Southwestern States Mission” series (homepage) examines mission life in (mostly) Texas around 1900.
 I’m posting late because of the crazy hedonism of the MHA weekend. Congratulations to Matt and Andrea for organizing a great conference. The post below was prompted by a conversation between sessions with Sarah Reed, a PhD candidate at Wisconsin in German Literature. She commented on some German speakers mentioned in an earlier SWSM post and also gave me some context for German-language sources I’ve been encountering. (Thanks, Sarah.) I count 71 diary entries that refer to “Germans” out of about 4,500 entries. Ten refer to “Polanders” and three to “Bohemians,” though there is some overlap (ie, some entries refer to more than one identity).
 I am not certain that missionaries writing about “Germans” were making precise linguistic, ethnic, or national identifications. For starters, I assume that most, if not all, of those they encountered were US citizens (in some cases, for generations). Second, the diaries mention “Polanders,” “Bohemians,” and “Germans,” but my spidey sense says that there is a chance that some of the “German” might have referred to Polish, Czech, or Slovak identities or languages. There is one instance when Duffin baptizes someone he identifies as “German” but gives the birthplace as Poland (which is hardly unusual for German/Polish genealogy).
 Brooks, 1900 Mar 02 Fri.
 In addition to the one already cited: “We went on to another house and asked for shelter. We were refused, also three times after that. We finally talked a Polander into the notion of taking us in. He said we could sleep in the corn crib, but they made us a bed on the floor. Before we went to bed, Elder Huntsman went out side. These people had a very bad dog. They kept him chained most of the time, but had turned him loose that night. He came very near taking Elder Huntsman down. Would have done so if the people had not rushed out and kept him off. When he came into the house he was pretty well-scared. The name of the Polander was Tom Owerabrok. None of the family could talk but a very little English.” (Brooks, 1900 Mar 22 Thu). “It had begun to get late so we began seeking for entertainment. Were refused 6 times. Went up to one house and I had to fight the dogs away with my umbrella. Could not stay there. Next house the wife was sick. Next place, a big white house, had company. But, finally we got into a poor German’s house. He could not understand English. Had a nice supper of German sausage and a good bed.” (Jones, 1900 Dec 05 Wed).
 Brooks, 1900 Mar 06 Tue. There are also some mentions of sausage, in one case specifically identified as “German sausage.”
 Brooks, 1900 Apr 13 Fri. I have not compared encounters with German preachers and other types, but the diaries include multiple discussions and arguments with preachers identified as “German.” Some of the Germans are identified as Lutheran and others as Catholic. An example of the (consequences of) cluster migration: “The German people were thick as usual. We could do but very little, only give them a tract.” (Brooks, 1900 Mar 05 Mon).
 Folkman, 1900 Oct 08 Mon; Jones, 1900 Dec 12 Wed. There are many examples. One more: “Continuing our walk, we began to seek for a place to tarry over night. The first man we asked was a German. He misunderstood me. After I got through asking him, he went into the house and came back with a dollar. I looked at him and he said “If it will do you any good, take it.” I said we don’t want that. We want to stay all night. He said yes, we would be perfectly welcome to stay but they were all going off. I sold him a Voice of Warning. Went out to Bro. Jund where we stayed over night. I gave him a good talk. Held prayers and retired.” (Jones, 1900 Dec 11 Tue).
 “The roads had dried some by this time. We went to a German preacher to get entertainment. When I told him who we were he nearly fell over backwards. He could not keep us but sent us to Mr. Sheef, who belonged to the Evangelical Church. He gave us a nice supper. I gave him a tract to read that was written in German. He read a little then came to the word Mormon and asked if we were those Mormons that lived in Utah. He said that he didn’t believe in such doctrine of having so many wives. I tried to explain but could not make him understand much” (Jones, 1900 Dec 30 Sun).
 Brooks, 1900 Mar 30 Fri; Brooks, 1900 Mar 19 Mon. The next morning: “Had a very good breakfast with that Polander. After eating we again started on our way. We went about three miles and came to an American place.” (Brooks, 1900 Mar 20 Tue). There are multiple examples. Just two more: “At night we came to a German’s where we were kindly taken in and given a nice supper. Had a good talk.” (Jones, 1901 Apr 27 Sat); “We went to an American place. He asked us in his house. I asked him if it would be agreeable for us to stay all night. He says yes I think so. After we had been there a short time talking with his father-in-law, the man decided his family was sick and they couldn’t keep us so we had to move on. There were a number of American families living close there but we had to stop with a German. It seemed as though we couldn’t stop with the American people.” (Brooks, 1900 Mar 30 Fri).
 Duffin, 1906 Mar 3 Sat. By 1906 the mission name had changed from Southwestern States Mission to Central States Mission. “Went tracting, visiting about 25 families. It is hard to do anything in this county unless we could talk German, then it would be a good field.” (Folkman, 1900 Oct 11 Thu); “Copple Settlement, Ind. Terr. … Reports of Elders: … Elder Wm. W. Tingey… Have some German people investigating the gospel. After the busy season set in it was hard to do much among the people. Many German people in Kay Co. principally Catholics.” (Duffin, 1904 Aug 31 Wed); “[Kansas] Elders report fair progress being made, but in some parts many German people with whom they could not converse on account of them not understanding the language.” (Duffin, 1903 Mar 1 Sun).