Which authority regulates airliner design?
The safe operations of an aircraft is called airworthiness. It is the standard by which an aircraft is determined fit to fly. Responsibility of airworthiness lies with national aviation regulatory bodies, manufacturers, as well as owners and operators. International standards are dictated by ICAO during the aircraft design process:


  The International Civil Aviation Organization sets international
  standards and recommended practices for national authorities to base
  their regulations on


FAA has regulations governing all aviation activities in the US. EASA also has airworthiness regulations.

The answer to your question is:


  The aircraft manufacturer makes sure that the aircraft meets existing
  design standards, defines the operating limitations and maintenance
  schedules and provides support and maintenance throughout the
  operational life of the aircraft.