Dark mode is finally coming to Google Translate on Android and iOS devices
9to5google reports this is a server-side switch, so you'll have to wait for Google to make the new feature available on your device.
However, if you're on iPhone, the latest version of Google Translate that appears in the App Store should add dark mode support. It even looks slightly better than Android users are getting at the moment.
The dark gray background colors in Google Translated for Android looks different than the dark mode implemented into other Android apps. The same goes for the text colors, which seem a bit washed out. Perhaps this version of the dark theme needs a bit more polish before primetime.