When can I use the summer tires of German cars?

Mainly depends on the temperature. Theoretically, if the road surface temperature is higher than 7 degrees, summer tires can be used. If you look at the solar terms, it is also after the vernal equinox and during the period between cold dew.

The main difference between summer tires and winter tires is the pattern of tires and the composition of rubber compound. The pattern of winter tires is suitable for the ground with snow and frost. The rubber is soft and the composition is resistant to low temperature. In summer, the tire rubber has high hardness and high temperature resistance, and the grip of the pattern on the stagnant ground is higher than that of the winter tire.

Actually, any car is the same. Tires are the "shoes" of cars. People can wear different shoes according to their occupation, but they don't wear shoes according to their nationality or body shape.