What do you thing is the best gear-change for the big intercontinental bicycle-trip?
Which gear-change is the most stable one? Chain-gear-change or nave-gear-change? In which countries i will get which replacement-parts, Where i will get replacement-parts for nave-gear-change? How many gears do you need for your trip?
Thank you for your answers.

The simpler, the better.
E.g. I would recommend primitive thumbshifters, no STI levers.
The rest depends on your purposes, but I think that a standard three-ring-eight-cogs setup will be the easiest to be repared in any corner of the planet
OTOH you could rely on the (allegedly) undestroyable (and nearly unpayable ) Rohloff gearbox - some of us do so.

What is "NAVE GEAR CHANGE"? Where are you comming from, did you learn English at school? Maybe you´ll give it a try at the currency change. (yoke)
If you look for a multispeed setup on your bicycle (single speed, internal geared hubs or derailleur systems with up to 64 gears) try www.sheldonbrown.com
Aku