Currently, there is no worldwide treaty governing animal welfare. There have been a few proposals to the UN throughout the past few decades, but none have been undertaken. There are some European Union (EU) treaties related to animal law that the U.S. is not a party to, and those can be searched for on Eur-Lex.
In researching animal laws from other countries, it is always a good idea to consult USD Law's Foreign Law Guide, which guides you through the process of searching for laws of other countries.
Another helpful resource is Georgetown Law's International and Foreign Animal Law Research Guide, which provides thorough information on several country's animal welfare laws, as well as other resources.