The State is already encouraging people to have families. In France you receive money every month for each children you have, you also have tax reduction.
The Nazi propaganda that you're refering to isn't an immigration problem: France brought many africans from the colonies into the army but in specific batallions going straight forward to the front line. Those who survived were sent back to their home as soon as their service stopped.
The African immigration to France really started after WW2:
First wave: 60's Maghreb populations (Morocco, Tunisia and Algeria which was a a french region and not just a colony till 1962). But also thousands of Portugueses, Italians, Spanish and Polish (even if most Italians and Polish immigrants came before WW2 to work in mines and factories). The European immigrants represented around 75% of the newcomers.
Mainly construction workers coming to "rebuild" France during the golden years of quick economic expansion and modernisation of infrastructures.
These guys stayed, settled and had families.
in the 70's/80's after the second oil crisis the economic growth slow down and at the same time the black african immigrants arrive. The Europeans are less intrested in coming here so the surburban districts, built and designed to host newcomers, are filled with less diverse populations than it used to be and turns into ghettos, isolated from the rest of the population. The social gap increased decades after decades till today. Result is now that we face a phenomenom never seen before since the French Revolution, communautarism.
The French Republic isn't recognizing any race or religious difference as for it, you're either french or you're not. All the rest is up to you and shouldn't drive your life. Your main identity is your citizenship and we're all equal and brothers under the protection of the republic etc ... But now we have people looking for specific identification of their group, themselves or other communities, discriminating etc ... Some muslims refuse to mix with other groups, integrate women into society, ... Some right wing groups claim catholic heritage and white supremacy, or cultural priority. In some districts you'll find only people coming from country XXX, etc ... Royalists and catholic refuse to apply to some republic rules, bringing god in the conversation, etc ...
So now we're trying to defuse that bomb as best as we can, but it might explose one day or another.