I've Googled "tooth fairy traditions" for Germany, Japan and "international" and also "tooth traditons Germany". In Japan, apparently it is traditional to throw a lower tooth onto the roof, and an upper tooth into the floor. Is this still in practice?
I haven't seen any mention of Germany in various tooth fairy traditions. I've seen a couple message board replies that seem to suggest they don't do anything special with the teeth there. However in nearby Austria, apparently they have the same tradition of roof/floor as Japan. You can also make the tooth into a pendant or key ring. Does Germany have the same traditions as Austria, or do they just...not have any specific tradition?
Edit: My character is from North Rhine-Westphalia in western Germany, and it's been pointed out that people in other parts of Germany might not know/understand the traditions of parts nearer to Austria. So there's that. ^^
Or have/did either of them adopted the Western "money under the pillow" tradition?