You don’t have to alternate between languages for interpreter mode to work.
Interpreter mode is already on your phone there’s no app download required. For instance, you can say, "Hey Google, be my Spanish interpreter" to start Interpreter mode and get real-time spoken (or, on smart displays, written translation) to aid the conversation. Just ask Google Assistant to help you conduct a conversation in a language.
The feature has since rolled out to Android and iOS phones, as well as the lineup of Google Assistant devices and smart displays.
Google first unveiled Google Assistant's Interpreter mode at CES 2019. (Pocket-lint) - Google Assistant has a huge variety of convenient features to help out with your day-to-day life but one you may not have tried before is Interpreter mode.