I have a driver's license from Brazil.
First, you can obtain an international drivers permit from your home country. If you have a valid Brazilian licence and an international drivers licence, you are allowed to drive in Germany as soon as you step out of the airport.
You only need a German licence if you take residence in Germany or if your international licence is expiring and getting a local German one would be cheaper than flying back to Brazil to get it renewed.
If you do take residence in Germany, there is a chance to have your Brazilian licence transferred to a German one. But Brazil does not seem to be in the list of exemptions, so you'd need to take both practical and theoretical tests, you just don't have to take the lessons. As you will surely fail at least the theory test without extra practice, you could as well do the regular German licence.
That said, a German drivers licence tests can theoretically be taken in days (theoretically even in one very long day if you are pure Genius). You need to take a theory exam and a practical test. In addition you have to have three mandatory, multiple-hours special lessons with your instructor (night driving, Autobahn and Landstraße). At the end of the last, practical test, the tester (a third person from a German authority, not your private school instructor) will hand you the new licence papers right in the car.
There are driving schools that specialize in teaching all the material in two weeks or even one week. They target school kids that want to do that in the holidays mostly, but they will teach anyone who can pay their price.
If you have friends in Germany, your best bet might be to take some time and have them explain the differences and drive around in a car on your own for a while. Cramming that all in your head in just one week is not easy. Normal Germans do that when they go to school like once or twice a week for an hour or two and take months. As always with theory tests, just being able to drive here does not mean you can pass the theory test. I did some of the practice tests for fun with my non-German wife to see all the differences between her home and Germany and I have to say I would have failed it a few times as well. Not in any obvious way, like not knowing who's right of way it is, but it's been 20 years since I knew how to calculate the distance to stop at a given speed. Nowadays kinda feel it... without actually knowing it. And it's probably the same with you if you have had your licence for a longer time.