Why Do Mr. and Ms.Frank Speak German In Scene 1-4 Act 1 ?

I believe this is found in Scene 4. The Franks revert to speaking German at an emotional time where they're having problems with Anne. It's a nervous change...... think about someone you know who does not speak English as their first language...... when they're upset, excited, or emotional they tend to revert to their native tongue. This is the reason the Franks speak a bit of german in this scene.