RTranslator is the world's first open source simultaneous translation app based on Google's APIs.
Connect to someone who has the app, connect Bluetooth headphones, put the phone in your pocket and you can have a conversation as if the other person spoke your language.
To use this app, you have to configure it. Also, the Google APIs on which the app is based are not free. When you create your account for the first time, you have a $ 300 voucher valid for one year, but after that, the APIs will no longer be free. Read the GitHub page for more details and a tutorial for the configuration (https://github.com/niedev/RTranslator).