what does it mean by tracing their fish hooks , as if that was french too.

The reference is made to the pigeons on the roof top who were tracing their feet, which seem to resemble fish hooks in their appearance, on the surface of the roof in a manner that was like writing. To Franz, it seemed as if that even the pigeons were writing in French with their fish hooks and were aware that from the next day onwards, they would also be forced to change their language from French to German.