I'm sure that the voice control would be dependent on the language settings of the iPhone itself. Say, if you set it to English, saying "Nana-Roku-Roku-Shi-Ichi" would do nothing, but setting it to Japanese would yield "7-6-6-4-1".
I agree with Aries Wren. The language settings are up to you, so as long as that language is one of the language options, then you should be able to use voice commands in that language.
German is one of the major languages and you won't be having any problem with the voice control. It all depends on your voice control settings and I am certain that German is one of the option available in the set.
