Earth Made of Glass — John Barnes

Welcome to the Thousand Cultures--in which humanity's hundreds of settled worlds are finally coming back together, via the recently invented technology of instantaneous travel. And in which Giraut and Margaret work as professional diplomats, helping to finesse the stresses and strains of so much abrupt new contact among wildly diverse cultures.

Now, however, their task is to bring in the terrifyingly hostile world of Briand, a planet of broiling acid oceans whose only habitable portions are Greenland-sized subcontinents that project out of the abyssal heat of the planetary surface into it stratosphere.

But Briand's physical hostility is nothing compared to the venom its two human cultures bear toward one another. Into this terrible world come Giraut and Margaret to try to do the right thing by the Cultures, by the inhabitants of Braind, and by one another.