If you were previously using SwiftKey Cloud (4.2) or 4.3 beta, just sign in to your SwiftKey Cloud account after upgrading and all your learned language (from the beta and/or your market release SwiftKey) will be combined and saved into your SwiftKey Cloud account.
Not previously using SwiftKey Cloud or 4.3 Beta (or later):
Upgrading to the latest market version of SwiftKey should not affect your learned language.
If you decide to sign up to SwiftKey Cloud, all of your learned language will be saved to your SwiftKey Cloud account and from then on you should never lose your language again. See the article, ‘What is SwiftKey Cloud?’ here for further information.