The country of origin doesn't matter for some race series, this is not a German only event, this is the DTM event, therefore any car that meets DTM specifications and requirements can enter.
The Alfa Rmeo 155 is Italian, but it is a DTM car that actually raced in, and won, the DTM championship. Supposedly, the Lexus was built to DTM specificaions so it therefore qualifies for the DTM race on GT5.
Races on GT5 are to do with country of origin or specific make or type of vehicle, it varies throughout the game.
However, there are some exceptions, like the Japanese GT event (JGTC or Super GT) in which supposedly (I haven't tried) the Minolta Toyota can race in, which is absurd as this is a Le Mans car and massively more powerful and lighter than the JGTC cars.