What if I fail to receive FAA Medical Certificate via mail?
You're really asking a postal service question (and from my experience with the postal service you're probably out of luck, even if you had the tracking number: Unfortunately when the USPS hands packages off to other nations' postal services that's often the last tracking update - either the item gets to its recipient or not, but it's gone into a black hole).

In any case the answer to the aviation/FAA half of the question ("I haven't gotten my new medical certificate yet - what do I do now?") is much simpler, and involves more helpful people than international mail: "Contact the Medical Certification branch" (see here).

In this particular case you want to actually call them on the phone and speak to a human being to determine (a) if they have a tracking number, and (b) how to proceed to ensure that you have a valid medical certificate in your possession.
I know in cases like this the Medical Certification branch can also fax you a copy of the certificate (I believe these are valid for 60 days) while you try to get things straightened out and get the actual paper certificate in your hands.