I am from Poland. This past weekend, I talked with my mother-in-law, who was born in 1935 in France. In 1939, right before World War II broke out, she and her family went back to Poland. I was shocked and asked why they did it. Weren't they aware that Germany could attack Poland? She answered that they were guessing that it may happen but, in case it did happen, they wanted to be eventually on the Polish side. Moreover, she said that her father's brother stayed in France and, when Germany attacked France, he had to fight on the first front. Is it true that France first used people from other countries to fight before starting to use French people?
No, not true.
France had conscription. In case of war the army would be placed on alert and war footing, meaning that draftees were recalled to the army. Foreigners cannot be drafted. I know of no nation that drafts foreigners. That's technically next to impossible: which foreigners? Tourists? Expats? How well do they need to speak or understand French? (Foreigners who volunteer is an entirely different topic.)
Your mother may be mistaken by the French Foreign Legion. The Foreign legion recruits foreigners exclusively. Frenchmen can enlist, but must do so under a different name and nationality to serve in the ranks. All officers are French. Nearly all non commissioned officers are promoted from the ranks.
It is true that the Foreign Legion was founded to save spilling of French blood. A legionnaire who gets seriously wounded in action gets French nationality immediate. Otherwise, he can become French citizen after 3 years of service. The legion was specifically set up to save French lives in colonial campaigns, but they also fought in France itself during WW1 and WW2.
A little side note: the Dutch East Indian Army KNIL was set up for exactly the same reason: to save, in this case, Dutch lives by recruiting foreigners to do the job. The KNIL was trained in The Netherlands, but never fought there.
It's very likely your mother talked about the Polish army in France, who were refugees from Poland after the occupation by the Germans in 1939. Those were volunteers, the unit was not (as the Foreign Legion was) set up to save French blood.
Another event that could be simplified into this idea was the deployment of French colonial troops in the early stages of WWII (and WWI). France had a significant army in Algeria from 1830 until 1962, which contained large numbers of Algerians.
At the start of each war, troops from Algeria were shipped to France. This made perfectly good sense: France was threatened, and the units that were sent there were already in active service. That meant they could be ready to fight in France more quickly than French reservists could be called up, formed into units and got ready for action.
World War II and World War I are two separate conflicts that occurred at different times. France was involved in both wars, but I'll provide some clarification:
World War I (1914-1918): France was one of the major Allied powers during World War I. They did not send people from other countries to fight in World War I; instead, France and its allies had their own armies and conscripted soldiers to fight in the war. Many soldiers from France's overseas colonies, such as Algeria and Senegal, also served in the French military during WWI.
World War II (1939-1945): In World War II, France was one of the early victims of the war when Germany invaded in 1940. After the fall of France, the Free French Forces, led by General Charles de Gaulle, were established. These forces included French troops who had managed to escape the German occupation and continued to fight alongside the Allies. France also had colonial troops and units from its overseas territories who contributed to the war effort.
So, in World War I, France did not send people from other countries to fight, but in World War II, they had a diverse set of forces fighting for the Free French and also had colonial troops and units from their overseas territories involved in the conflict.