German is a good target for internationalization for three reasons. First, there are many potential customers. Second, German has a different set of grammatical rules and includes gender and other modifications. And third, the language has compound words resulting in generally longer than English strings. Working with German is a good test to how well your UI adapts to long strings as well as a good source of possible downloads.
Before you start adding German words, make sure doubling of strings is off by unchecking the box next to the command-line argument in the scheme-editing pane.