Is an international patent of software design possible?

There is no single patent registration that will protect an invention around the world. You still have to register your patent in each country according to the rules of that country. However, there are some entities that are moving toward multiple-country registration with just a single submission. Check the rules of the countries you plan to market your product in.

There is no actual International Patent - only an International Patent Application - filed under the Patent Cooperation Treaty (PCT). The PCT Application allows you to file one application, which is active up to 30 months - and thereafter make formal patent filings in the various PCT countries of interest. At last count - 148 countries are members of the PCT system. Good Luck!