Do all Amish speak German ?
If the Amish were to travel to Germany they would have a really hard time trying to understand conversations. Church services and the Bible they use are German but other than that they speak in what is know as Pennsylvania Dutch although some people refer to that language simply as Amish, Deitch, or Pennsylvania German.
There are some Amish that speak with a more Swiss dialect and the two groups Swiss Amish and mainstream Amish can not understand each other and have to speak English if they want to have a conversation.
Pennsylvania Dutch is for the most part an unwritten language and each community has some words that leave other communities scratching their heads. Even families in the same communities often have a word or two that seems to belong exclusively to them. After LV and I got married there were words that we had to decide how we would pronounce since his family had been used to one way while mine had pronounced it differently. I changed quite a few pronunciations and thankfully some of his weirdest ones were changed as well.