Finnish as a world language?
It should be Japanese, of course!Japanese can be written using either the Latin alphabet, or Chinese characters.  The two most common writing systems in the world.  It can also be written with its own elegant and purely phonetic writing system.  There's even Braille, Morse code, and sign language encodings.  It is truly media agnostic.Japanese has a regular grammar.  From a linguist's perspective, aside from the politeness system, it's really quite normal for a language.  Very little in it to surprise a Finnish, Turkish or Korean speaker.   (Unlike English, which if first discovered today spoken by a people in the interior of New Guinea, would lead to accusations of a linguistic hoax.)Finally, some 40% of Japanese vocabulary is based on Chinese, and Chinese and Japanese technical terms flow freely between the languages to this day.  Another ~20% of Japanese vocabulary is borrowed from European languages like English or Portuguese.  The majority of the world already speaks a significant amount of Japanese and they don't even know it!  For this reason, Chinese, European, and American, alike, usually find it quite easy to learn.