How do I activate live translator?

To activate a live translator on your phone, open Google Translate and tap Conversation, or on iPhone open Apple's Translate app and choose Conversation there. Pick the two languages, grant microphone access, and both apps translate speech as it happens.

To get your phone to translate speech automatically, Google's mode listens for both languages and figures out who is talking. Apple's works the same on-device for downloaded languages.

Those cover talking with someone. If what you want is a translator for your own voice while writing, activate it at the keyboard level instead: dictate in your language, and the text arrives in the target language, inside any app.

That keyboard-level option is voice to text translation.