Comment on the change of language in the novel.

Chinua Achebe's Things Fall Apart is as much a journey for its author as it is for its characters. Achebe was Nigerian, and he was born during the British colonization of the country. He helped fight for Biafran independence, although he ultimately felt frustrated by the Nigerian government, and spent many of his later years in the United States.
Achebe was an Igbo chieftain and spent his life working toward preserving his heritage. Like many other African authors dealing with the wake of postcolonial life, Achebe struggled with whether to write his works in English or his native language of Igbo. To write in English would be to speak in the tongue of his colonizers and to let go of his Igbo heritage. Conversely, writing only in Igbo would greatly limit the number of those who could understand his works.
In Things Fall Apart, Achebe opted to use a mixture of both languages. He wrote the novel primarily in English so that it could be consumed and understood outside of Nigeria, while using Igbo terms and phrases to capture the spirit of his culture.
https://www.newyorker.com/magazine/2008/05/26/after-empire