Translation message extractor got its first stable version. It is a command-line tool that allows automatically extracting translation IDs from PHP source files and writing them to
one of the translator message sources.
Such message sources are a good raw material for the actual translation. You don't have to keep track of what messages are there in the source code since you can re-run ./yii translator/extract command once again to update the message source.
You can find more details in the package readme.