Why did France decide to speak french?

France did not decide to speak French.

Countries do not decide what language to speak; languages develop. The people of ancient Gaul spoke various languages, mostly celtic; then they became part of the Roman Empire and learned Latin; then they were invaded by the Francks, who spoke their own Germanic dialect. Finally, the mixture of Latin and Frankish produced French (with some influence from the more Italianate dialect of the south).