language file errors during upgrade from 2.1.4 to 3.0.0

Update: so, just for testing I placed an empty file named en_lang.php in /application/language/en/ and the error went away.

I saw nothing in the documentation indicating that this should be necessary. What changed. I'd prefer to understand why it is that CI 3.0.0 is requiring the file en_lang.php or [idiom]_lang.php to be there.