adjective
BAD : Solutions to the country's interior problems are still a long way away.
GOOD : Solutions to the country's internal problems are still a long way away.
◆◆◆
interior = on the inside of a building, room, vehicle etc: 'The interior doors are still sound but the exterior doors need replacing.'
the interior = the part of a country that is a long way from the coast: 'The convoy set off at dawn for the interior.'
internal = involving what happens within a particular country: 'Each country has the right to control its own internal affairs.'