Hello!
Number 2 is easy, go to options on the main screen (not in a career) and set the speech to German. I play fully German language and I don't (or didn't) speak or read a word.
Not really sure how much knowing "Aller Maschinenhalt" or "Sehrohrtiefe" will help if I ever visit Germany, but it can't hurt.
I strongly (and I can't express how strongly) recommend Thomsen's Sound Pack, it is just an awesomely put together collection of sound files for SH3 and really increases the immersion. It takes a while to install in JSGME, to the point of looking like JSGME crashed, but 5 minutes of your time installing the mod makes the game so much better.