Yeah - there are a lot of countries that use English as a backup language. Yep - there is a lot of English around the world where the native languages has merged with English. These are the sort of English dialects that native UK or US people would have trouble understanding. One of these examples is the creoles language in the West Indies. Having said that, I've encountered people in the US that I simply could not understand.
The most interesting question about English is the idiom and slang used across the world in differing dialects - and a word in one dialect will have a completely different meaning in another. Hey - it trips up native English speakers!
If a person is looking to learn English - I always say don't worry about the various English dialects, because most native English speakers can change to more understandable version.