There's no difference between German phone lines and American ones, other than the plug and the tones (e.g. the ringing and busy tones, which sound different).
As for DSL... there is no one DSL standard, so even within the U.S., DSL modems are largely not interchangeable. The chances that your U.S. DSL modem will work on their system is slim. Besides, strictly speaking, it'd probably be illegal, since all equipment that one connects to a phone line in a given country needs to be approved by that country's telecommunications regulatory body.
Ethernet is the same everywhere.