most important unistate language

If you're willing to exclude relatively recent disapora, Hindi
Otherwise, Bahasa

Italian is not possible because is one of the three official languages in Swizerland.

There is no such thing as a unistate language. No language in the world is only spoken exclusively in one country.

Mandarin is not unistate, it is spoken in China, Taiwan, Singapore and Malaysia.

Hindi is not unistate. It is spoken both in India AND Pakistan where it is known as Urdu.

Flemish is not unistate, it is spoken in both Belgium AND the Netherlands where it is known as Dutch.

Flemish also causes a lot of confusion. Historically it's the language spoken in French Flanders, West-Flanders, East-Flanders and Zeelandic-Flanders and it's very simular to Zeelandic.
However, the whole Dutch-speaking part of Belgium is called Flanders nowadays. This has been so since the Burgundians unified it with Brabant. People call our variant of Dutch Flemish, while it's actually Belgian Dutch. The latter implies Belgian nationalism too much, and that's why people prefer to call it Flemish. Belgian nationalism is actually French nationalism in Belgian sponsored by British and French, life was better under Dutch rule for Flemish (even though few realize that nowadays).

Flemish is actually a regional Germanic language simular to Dutch. They even conjugate their words for "yes" and "no" (West-Flemish does it).
Flemish is nowadays being called West Flemish in Belgium and just Flemish in French Flanders (where nowadays only elders and identary Flemish speak is, France sucks at the language department).

And yes, Japanese is the most important unistate language. This is obvious.

But Singapore does count. Therefore, Chinese is off the list.

I think what was meant, was a language that is only official in one country... like Lithuanian. (Although this is just an example, not the most important one).

Yeah, be sensible about it, this isn't mathematics. For example Italian may be spoken by a few people in Croatia and be official in some minor provinces there, but that doesn't really mean it would rule Italian out for that reason.

I say, let us judge the language to be spoken in a particular country IF:

- Spoken by at least 10% of the population
- Official language
- It is the language of government/official proceedings, or used in conjunction with other languages as long as it is still widespread
- There is high usage in the media

I don't think this kind of language existed or will ever exsist.
http://hellomandarin.com