Gmail will automatically prompt you to translate a message if the language doesn't match your preferred settings. The translation feature for Gmail is exclusively for Google Accounts and doesn't extend to messages in IMAP accounts.
Important: At the top, you can find a banner that prompts you to translate when the message doesn't match your preferred language settings. You can manually translate the message after you dismiss the banner. You can turn off this banner's automatic translation for a language by opening the settings menu.
Translate a message
- Open a message in Gmail.
- Tap More
Translate message.
Translate message.