Going to a country you've never been? Bring along someone who has. Failing that, talk to another pilot who has been. Failing that, have a good handler with inside information about the ins and outs of the airports you are visiting.
While things are definitely becoming more standardized, there are still very big differences between many countries on how airplanes are expected to fly. The International Civil Aviation Organization (ICAO) sets out a game plan for all to follow, then allows everyone to change whatever they please, provided they publish the differences in their own Aeronautical Information Publication (AIP). How is your Nepalese? You would have to be able to read it to find out the flight levels in Nepal are based on quadranteal altitudes below 13,500 feet. What's an international pilot to do?
If you haven't been to a region in a few years, it really pays to talk to someone who has. If you don't speak the language it pyas double to have a contact at the airport to make things run smoothly for you, a good trip handler can be invaluable.
Keep in mind things are constantly changing, but I've included notes about navigation performance, RVSM, altimeter procedures, and ICAO differences for the various regions in a list below.
Figure: LFPB Airport Pages, from Jeppesen Airway Manual, Le Bourget, 30-1P through 30-9C
The Jeppesen airport pages are a must read, for starters. While for many airports there is little to learn there, for others the information is critical and you won't find it any place else. At Paris Le Bourget, for example, this is the only place you have access to, unless you have a French AIP, where you will find out the normal landing runway, the reverse thrust rules after 2215L, and a unique rule that allows you to intercept an ILS localizer without ATC instructions under some circumstances. Since these pages are updated frequently, every time you go to a particular airport you need to look at every page!
Figure: LFPB State Pages, from Jeppesen Airway Manual, France, Various.
The "State Pages" in the Jeppesen airway manual refers to the following tabs:
While they aren't always perfectly up-to-date, they often present your only source of need to know information.
Figure: ICAO Doc 7030 Cover Page, from ICA Doc 7030.
ICAO Doc 7030, Regional Supplementary Procedures, Fifth Edition 2008, and ICAO Doc 7030 Amendment 1 8 January 2209, are the official publication of regional differences between individual countries and the ICAO standard. In theory this should be your "go to" source, but it is woefully out of date. Nonetheless, there is good information to be had here.
Of course there are many. Here are some that I use:
I have summarized the regional differences, grouped as they are in Doc 7030: