Don't know about other countries models, but US models have a placard in the door jam or behind the glove box or center bin cover that contains the manufacturer's recommended tire pressure, and they are usually fairly low to give ride comfort, but may contain additional recommendations for high speed driving. Each country probably had their own homologation requirements for recommended pressure and how it is displayed.
Alternatively, you could check your owner's manual to see if recommended tire pressures are contained there.
As a final sanity check, find the Max load @ max pressure placarded on the tire and compare it to the normal weight of the car and use common sense as to what pressure is sufficient to carry the actual load that the tires see in service.
Chances are that 28 psi is adequate, but very high ambient temperatures or high speed driving would call for higher pressures to maintain a reasonable safety margin. Also, lower pressures will reduce steering response.
One should never go below OEM recommended pressures but higher cold pressures - up to the maximum placarded on the tire will yield better handling, lower fuel consumption, and increase tire life but the ride will be harsher. The recommended pressures for the OEM 185/65VR-15 tires on my 190E 2.6 are 23/29, if I recall correctly, and the car drove horribly. To mitigate the understeer and give decent steering response I ran them at 42/36. The were placarded at 44 psi max cold pressure. I run 36/32 on the currently installed 205/55ZR-15 Dunlop Sport 8000s on 6.5" 300E wheels.