You can install Microsoft's natural voices, which will be able to work offline. After installation, the voices will be available in any application that uses SAPI 5 (for example, in the Balabolka application).

English (Nigeria) voices: Microsoft Abeo Native (male), Microsoft Ezinne Native (female)

Official website of the Balabolka application:
https://www.cross-plus-a.com/balabolka.htm

The Windows Language Code Identifier (LCID) Reference does not contain a unique Language ID for English (Nigeria). Therefore, the Language ID for English (United States) was used.

---

It is necessary to install NaturalVoiceSAPIAdapter to use Microsoft Online and Microsoft Native voices. Download the latest version of the utility (NaturalVoiceSAPIAdapter_x86_x64.zip):

https://github.com/gexgd0419/NaturalVoiceSAPIAdapter/releases

Unzip the archive into a folder (this folder cannot be deleted while the voices are in use). Run Installer.exe, activate the check box "Enable Microsoft Edge online voices", click the buttons "Install 32-bit" and "Install 64-bit".
