When the French were out exploring they came across St-Louis. St-Louis is right by the Senegal River. They then used it as a popular post to buy and exchange slaves with other people around the world. ( French explorer Auguste Chouteau).
Then the French took over Goree Island which was owned by the Dutch. The Dutch had to give the Island up because the French Maritime Expansion was on at the time, so there was no way for the Dutch to fight back.
When the Seven Year War was over the British ended up winning the whole country of Senegal. They named it Senegamiba. At that time Senegal and Gambia were joined as one country. The name Senegal comes from the begging of Senegambia (Seneg) and Gambia comes from the end (gambia).
Eventually the St-Louis slave trading post became so popular, that the two French men Auguste Chouteau and Pierre Laclede, who ruled St-Louis, turned it into a city for people to live in. It was a popular city because of the trading post and its location which is near the Senegal River. ( French explorer Pierre Laclede)
After the American Revolutionary War, the French had won back its former colonies in Senegal from the British. The French then named what was Senegambia, Senegal again.
During the Napolionic Wars the British had stolen back the places they took from the French in the first place. Which had changed the name once again, from Senegal to Senegambia.
Eventually the French sold St-Louis to the American's and it became part of the Louisiana Purchase.
After the Nepolionic Wars the British decided to be friendly and give Senegal back to its founders: the French. Which then, the French changed the name back to Senegal, and abandoned Gambia.
Right after the French gained control over all of Senegal, they joined French West Africa; Which is a group of 8 colonies that were ruled by the French.
The French extended their power by taking over the rest of Senegal, so they had total controll now. Before they had most of Senegal, and the Spanish also had some too. But the French were able to steel it from them.
Eventaully the French had decided that Senegal was going to become part of the French Union too. The French Union was a group created by the French, its former name was the French Empire.
Eventually the French didn't want Senegal anymore. So the president of Senegal, Léopold Sédar Senghor and his people decided to keep all of their 'Frenchness' because they have been like this for so many years they were used to it. So they kept their name and their official language, French.