For consideration, a subscriber greeting, like any call handler, can accept caller input. You could direct the caller to press 1 to leave a message in German or press 2 to leave a message in English. Pressing either 1 or two would invoke a call handler that prompts the caller in the correct language, takes the message, and routes msg to subscriber's inbox. This assumes of course the caller has reached voicemail greeting due to busy or RNA.
Should be easy if you have routing rules based on calling numbers - the routing rule can "tag" the call with a language (any routing rule can do this) and connection supports multiple sets of greetings (one for each language installed) so if the call is tagged as german and reaches the user's greeting, it'll play in German - if French then the French greeting will play etc...
this is how the language sets in Connection are designed to work actually .
