If I buy an iPhone 6 in Germany, will it work in the U.S.?
As long as the phone you buy is factory unlocked (not a "no-contract" carrier phone), it should work. Same goes for phones bought in the United States: as long as it is unlocked, there shouldn't be a problem

One thing to keep in mind, is whether the models sold in Germany support LTE networks in the U.S. and on which carriers the phone can be used. It's most likely that the only major carriers you'll be able to use in the U.S. will be AT&amp;T, Verizon and T-Mobile, among a few smaller carrier. See this link from Apple that explained which models can be used on which carriers.