What determines how language in a particular country evolves. Why is French, for example, so different from German?

The determining factor of how language evolves is the archeological factors of the owners of the language. Any society that has similarities in their archeology will have similarities in their linguistics. Check for instance Chinese and Korean language, their sounds similar though not the same, but we can see some similarities in their linguistics, because it's archeology looks similar.

Using French and German as a case study. In August 843, the division of roman empire of Charlemagne into a western franconia realm which lead to the foundation of France and an eastern franconia realm with lead to the foundation of Germany, with are the original owners of the language, french and german. This two languages evolves from Rome, which has some similarities like in the case of gender, french and German has "masc, fem". In the case of nationalities their have " nationalitat, nationalite". Which are more similar, and many more.

But with time, things changes including our environment, behavior,and culture. This is the main reason why language changes too. For instance in France some part doesn't understand the language of the other, because things changes with time. French is far more different from German because it's been more than 110 decades when this two separate from Roman. From observational studies, it takes any language one decade to change in some structure.

French language is far different from German because the two language has been separated for more than 110 decades. If you check the American English and the British English, you will see some differences, because the two has stayed away from each other for more than 40 decades.

This changes is also due to technology, culture, and environmental factors.