I hit this too.
Seems a technical issue. An error message found in D3Debug.txt:
Detecting installed locales:
2019.09.06 02:15:14.257614500 Found: enUS
2019.09.06 02:15:14.257781200 Found: zhTW
2019.09.06 02:15:14.314018500 ERROR: Agent returned content with no "selected_locale"!
2019.09.06 02:15:14.314206500 FileSystemGetLocale, AgentSettings: 0
2019.09.06 02:15:14.314379800 Detecting first installed locale.
2019.09.06 02:15:14.314558500 Detected: enUS
2019.09.06 02:15:14.314712500 ERROR: Falling back to first installed locale.
The work around provided by @Hebalon works for me:
https://us.forums.blizzard.com/en/d3/t/game-always-in-english/2723/5