This new feature is only available if you are using the new Outlook on the Web (aka Outlook Web Access) interface (see https://t.co/wmPtK7ntgM).
When you receive an email which is not written in the language defined for your mailbox (see your regional settings), Exchange Online is now able to perform an inline translation.
You can define how the translation is going to work – from allowing complete translation or just some selected language (of the 65 supported).
To configure how the translation service is going to work, logon to your mailbox using Outlook on the Web (https://outlook.office.com) (ensure you are using the new interface, see the link provided above) and access your mailbox Settings using the Gear button and then click on the View all Outlook settings link available at the bottom
Then access the General\Message handling section and locate the Translation section; you can then disable the translation service or define a list of language for which the message will not be translated
Then when you will received an email in a different language, depending of these settings you will be asked to translate it or it will not be translated.
You need to have enabled the Conversation mode (available from General\Layout\Message organization)
