Yes, there are safe and approved ways to address this issue. Since your German car has a two-prong U.S. AC outlet and your computer's AC adapter requires a three-prong connection, you have a couple of options to ensure safe usage:
Adapter Plug: Purchase a suitable adapter plug that converts the two-prong U.S. outlet to a three-prong outlet. These adapters are commonly available in electronics stores or online. Make sure to get one that is compatible with the voltage and frequency of your car's AC outlet and is rated for the power requirements of your computer.
Grounding Adapter: Alternatively, you can use a grounding adapter or "cheater plug." This adapter allows you to plug your three-prong AC adapter into the two-prong U.S. outlet, but it won't provide a true ground connection. Keep in mind that while this solution may work, it's not ideal from a safety standpoint since it bypasses the grounding protection.
When choosing either option, it is crucial to ensure that the power requirements of your computer and the voltage supplied by the car's AC outlet match. You can find this information on the labels of both your computer's AC adapter and the car's AC outlet.
Moreover, before making any modifications or using adapters, it's advisable to consult with a certified electrician or an automotive expert to ensure that the setup is safe and won't pose any risks to you, your computer, or your car's electrical system. Safety should always be a priority when dealing with electrical connections.