What does the number of gears have to do with safety? And what do you mean by "needs of most countires"? Laws?

As far as I know, most local regulations require a rear brake capable of skidding the rear wheel on dry pavement. Many require bell or horn. Most require at least a headlight at night and a reflector at the back. Some places also want you to have spoke and pedal reflectors, some insist on reflective tape. Some have mandatory helmet laws.

These are too varied, but in general if you use common sense (have a good braking system, light yourself up at night), you'll be fine in most countries. Most of the time you will exceed the legal requirements. There might be some weird obscure regulations in places that a common-sense set-up will not quite satisfy, but I doubt you'll be hassled about them.