Inheritor [Foreigner 3] by C. J. Cherryh
This is the third book in the Foreigner series. It's a sequel to Invader.
The premises of the series are: About 200 years ago, a lost starship left a crewed space station orbiting an Earth-like planet. The planet was already inhabited by an intelligent basically humanoid atevi race. A crisis forced the space station crew to go down to the planet. Humans and atevi think and assume so differently that misunderstanding led to war. Humans lost and were isolated on one island, Mospheira. All contact between the two species is now through a single human diplomat / translator. Humans maintain a bargaining position with their more advanced technology. The unexpected return of human starship has complicated matters more.
The starship Phoenix is seeking help from the planet. They're hedging their bets by dealing with both the atevi and the human colony. Phoenix has assigned one liaison each to the atevi and the human colony. Bren is training Jase to be the starship's liaison to the atevi. Both Bren and Jase seem suspicious of the other's agenda.
To truly help Phoenix, those on the planet need to build spacecraft. With assistance from Phoenix, both the atevi and human colony are working on space programs, but the atevi program is pulling ahead. We eventually learn the Phoenix is worried about a threat from outside the star system.
Both the atevi and human colony are still infested with factions. And Deanna Hanks seems to be up to more mischief back at the human colony. Diplomatic matters become tenser when communications with Phoenix are lost.
Meanwhile, Bren finds himself increasingly identifying with the atevi.
Some readers will like it for its ideas, but others will consider it slow-going.