It is comparing Colombia and Lithuania. It means countries as different from each other as Colombia and Lithuania.

Think of it as "Colombia and Lithuania" as a single thing in this sentence. The sentence is saying that countries as diverse as these two countries (in this context, diverse means 'different' ") use Cambridge textbooks.

In this case, "diverse" means "different." Colombia and Lithuania don't have much in common, but students in both countries use still Cambridge textbooks.

I don't even know what the sentence means. It is impossible to know whether the author intended "diverse" to modify "countries" or "Columbia and Lithuania". Those are not the same. If it modifies "countries", it's saying the countries are different. If it modifies "Columbia and Lithuania", it means that each of those countries is a "diverse country", whatever that might mean. "Diverse" is a terrible word to use here.

To me it's not really clear. It could mean something like: ranging from ... to ... --> You could interpret it like this: Students all around the world use Cambridge textbooks, even in countries such as Colombia and Lithuania or it could mean that the author believes that people who live in Colombia and Lithuania are from a wide range of nationalities and backgrounds. I personally think the former is a better interpretation of the sentence, however.