Dialogue in Time Period...

I think nailing the dialogue from a certain time period is really hard. For example, would you say girl or lass? Mother, mom, ma? Dad, father, daddy? Contractions or no contractions?

Basically, I've been narrowing my eyes at my novel in anger, unable to figure out which words to use. I'm in the mid-1940s, in France. I got into a heated debate with a friend who's critiquing my novel, as he says he thinks the language I use is to informal for my time period. I argued that I don't think people really spoke formally in the 40s....and he said something that resonated with me. "I think you should in a way that your audience thinks the characters should sound like."

So, I'd like to start a thread where people have sentences and then a time/place reference and people can comment on if it "sounds right".

For example, a sentence I'm struggling with: "It's Vincent with the eggs, Mom!" Renée shouted.

Renée is 16, and this is 1942 France...so something tells me "Mom" isn't the right word to use. Would i"Maman" sound better?

In this case I would use "maman" instead of "mom", and for the father "papa". This doesn't even depend on the time period for me, but counts as a way to remind the readers that this is a French family we are reading about. The same way as I would speak about "rue Ste. Catherine" instead of "St. Catherine Street" when talking about location, just to give a random example.

I find it very important that the characters speak in a way that is appropriate to their time. Yet at the same time I try to avoid going crazy with colloquialisms and slang from the relevant period. Blatant anachronisms put me off (I've stopped reading books that had a perfectly good storyline but the author kept using modern expressions that did not fit in at all), but I've also read books where the author apparently accepted a challenge to use every single word in their dictionary of period slang, which almost made the book unreadable.

Finding the right phrase to express a time period is always a tightrope walk. One can neither modernize the expression (this wouldn't fit in time) nor use the phrases that was used that time (because the reader wouln't understand - e.g. I had to write in a vulgarized latin in that case). And yes, people in the 1940s did speak formally, as Aurora Ottiliana pointed out. Even born in 1967 I was used to adress anybody who wasn't part of the familiy with the formal "Sie" - and it's still appropriate facing a stranger. So, I'm German, but same counts for nearly all countries in continental Europa.
So my advice would be the same, Jack1761 had given: Use some typical french words to show, that the story is settled in France and avoid modern slang but try to find some phrases, that were typical for that time. In the given example I strongly would suggest "mamam" for adressing the mother and "fille" or "gamine" instead of girl / lass.

There would be a couple of important indicators for a setting in France during the Nazi occupation:
- Forms of address;
- Military ranks.

With the German military swarming all over the place, I would refer to German troops using German military ranks like Hauptmann, Feldwebel, Unteroffizier, etc. At the same time, I'd refer to French ranks (police or military) using period French terms or similar-sounding English words like Brigadier or Sergeant.

Forms of address - French vs German (Mademoiselle vs Fraulein, Monsieur vs Herr). The German occupiers would mix the German and French forms of address atrociously. French collaborators might use the German forms of address to appease Nazi bigshots.

I would throw in a few German words to show that it was during the war like Stuka for a Luftwaffe bomber or Opel-Blitz for a Wehrmacht truck rumbling down a town street. Such words typically became a part of the regular civilian lexicon in every Nazi-occupied country.

Also, the classic French derogatory slang for a German soldier - Boche.
Collabo (from collaborateur - collaborator) - slang for French collaborators.