If a foreign aircraft manufacturer wants to apply for a type certificate from the FAA, will the FAA accept the application if there are currently no buyers for this aircraft in United States?
ANSWER: Reference FAR 21.29 - Issue of type certificate: import products. You can access this regulation by clicking on the FAA Regulations Tab in the left column of this page.
There is nothing in the regulation that would prevent you from making application for a type certificate, even if you currently have no customers.
The rule says "a product that is to be imported into the United States." If your intention is to market the aircraft to the U.S., you should have potential U.S. customers before beginning the process.
There is a bilateral agreement between Russia and the United States that contains implementation procedures for airworthiness. You can follow this link to download a copy of that agreement.
The Russian civil aviation authority would have to certify that the aircraft met the requirements for certification as outlined in FAR 21.29.