Perhaps the most important factor in the program was the imaginative use of accents: the main characters (i.e. the Cafe staff: Rene (Gordon Kaye), Edith (Carmen Silvera), and the two waitresses all spoke English with French accents. All the Germans obviously spoke with German accents, but when Officer Crabtree speaks, since he is really English, he speaks a sort of 'Pidgin' French, which turns out, for example:
"I was just pissing the got"
"I was just passing the gate"
This demonstrates to the audience (who are presumably meant to be "French"), that the character has either not got a good grasp of French, or (more likely, to me at least) that the French don't have a good grasp of English, as when the French Resistance girl (Michelle?) speaks to the British Airmen, the dialogue will be spoken like the example above, in a posh British (Received Pronunciation) voice, to demonstrate that she is speaking English (as when she speaks French to Rene et al, it is with a French accent).
I'm not really sure how to make this less confusing...