"Google will announce the next step in Gmail's evolution, a new product with a European multilingual angle on Monday. At an event in Brussels to mark Gmail's fifth birthday, Google will look at the impact of cloud computing on how people manage their daily tasks, review Gmail's evolution to date and announce the next step in its progression, the company wrote in an invitation."
Most likely, the new product is a Gmail Labs feature that lets you translate messages written in a foreign language. I've found the screenshot that will be used for the feature, which will probably be another great use of the Google Translate API.
Similar features that use Google Translate have been implemented in Google Reader, YouTube, Google Maps, Picasa Web Albums, but the future is a powerful client that translates pages on the fly.
Update: Gmail Labs is now available in 47 languages, but the translation feature hasn't been released.