I myself have fallen victim to the urban legend that, at one time, German almost became the official language of the US. In hindsight, the idea seems preposterous. I think I may know why it sneaks through the sniff test on those of us who should know better: another surprising (I think closer to true) fact is that German ancestry is possibly the largest single category in the US. If one is already aware of that fact, the assertion about language may slide by the reality-checking circuits.
(I am not convinced that German is really #1, either as predominate ancestry or even as a component. I suspect that is English, but for historical reasons there is little self-awareness / self-identification of English ancestry--being a WASP is probably the proxy--it is very likely underreported.)