The Romans occupied Gaul (France) for centuries, so when the Germanic Franks moved in, they simply adopted the Roman language (Latin) of the Roman population that was already there. Now the Romans did occupy Germany, but it was only for a short time (a few decades IIRC), and they never Romanized it, so the Germanic tribes in Germany never came to adopt Latin as their native language. French is essentially an offshoot of Latin, like other Romance languages (Spanish, Italian, Portuguese, to name a few).