What would Langston Hughes think of race relations in America today?
It's difficult to say how Langston Hughes would feel about race relations today. Surely, he would be happy that some progress has been made. But in more specific reference to your question, Hughes was concerned with art and identity. He had said that his political views were non-theoretical, so his aim was objectivity. Unlike some of his contemporary African-American artists, Hughes didn't avoid stereotypes because he wanted to be as objective as possible, depicting real life.
He didn't want African-American identity, consciousness (or the art depicting it) to be some fad that whites could indulge in. Also, he did not want African-American artists to adopt/assimilate Western Eurocentric (or White American) styles, themes, and content. In other words, he was wary of the way the West has appropriated other cultural production and he saw the same danger of the reverse being true; of African-American artists adopting the techniques of the white majority.
That being said, in terms of social relations, I'd say Hughes would be hopeful but disappointed because race is still an issue. In terms of art, Hughes would be pleased with the amount of scholarship and art devoted to issues like Race Studies, African-American identity and Post-colonial studies. He would also be proud of African-American artists who come from poor backgrounds, become successful but give back to their communities.
As we've gotten further away from old stereotypes (and invented new ones) the idea of an essential African-American (or White American) identity has become more malleable. In other words, the white and black cultures have borrowed from each other (what Hughes was concerned about), but this has been more open and obvious in recent times.
It's hard to say, but Hughes might find this a conflicting development, a loss of what is definitively African-American and what is definitively White, thus making it more difficult to focus on the overt and subtle socio-economic conditions in which the definitively White class continues to oppress the definitively Black class. However, after considering the progress that led to such development, my guess is that he'd welcome this complication as the result of cultural collaboration, diversification, and a genuine movement to not just ignore stereotypes but create socio-economic conditions which make such stereotypes obsolete.
As an objective artist, Hughes would promote the progress we've made but simultaneously note we have miles to go before we sleep (or pat ourselves on the back for being "post-racism" - which we are not yet there).
It depends on how he'd see the two cultures interacting. Are they appropriating each other's culture or sharing?