Op made his question really clear and I feel many of us went a little over board.
The question is if a unified language can solve tribalism in our country, not whether tribalism can solve our bad economy.
China, literally one of the most developed countries in the world has over 700 languages and a former president of theirs forced everyone in the country to learn a particular Chinese language which was Beijingese . It evolved over time and became the standard Chinese language over 92% of their population speak. That didn't mean that the remaining 899 other languages were lost or suppressed . Today we still very much hear of the Cantonese, the mandarin, the taiwanese and many others, cause even in their unification they protected and preserved their culture.
.....sorry for the drift.
Anyways, the point I'm trying to make is that togetherness plays a very major role in the growth and development of a group of people. One crucial part of togetherness is to be able to communicate in the same dialect. To be able to speak another man's language doesn't mean your losing your identity, it just means your strengthening bridges and  relationship.
Today in china there is very much still diversification and unity which makes them strong, but in our country it's only diversification I see, no unity in any form. After graduating from the University we go for NYSC, which was established for the sole purpose of unity in diversification but after everything we still end of casting votes for people during elections that we know very much are incapable of the duty and responsibility of leading us, mainly because why? were of the same tribe, that's not unification at all.
So, in my own opinion a unified language will very much solve the problem of TRIBALISM in Nigeria and help us to individually stop categorizing ourselves under  tribes and just see ourselves as one Nigeria.