You make a good observation, but it can also go the other way. There are bright immigrant/expat kids who don't speak German well initially, but by being treated

You make a good observation, but it can also go the other way. There are bright immigrant/expat kids who don't speak German well initially, but by being treated the same as native speakers, they catch up by third or fourth grade sufficiently, and in addition also learn English to comparable levels (or better) as the German kids. This leads to good integration, and they would not be well-served by being put on a "dumbed down" track. The curriculum is by and large so light anyway (compared to e.g. Eastern European countries), that sharper kids coast through without much challenge.