language file errors during upgrade from 2.1.4 to 3.0.0

According to the CI3 documentation (http://www.codeigniter.com/user_guide/li...guage.html) I should be able to use the shorthand.

Internationalization

The Language class in CodeIgniter is meant to provide an easy and lightweight way to support multiplelanguages in your application. It is not meant to be a full implementation of what is commonly called internationalization and localization.

We use the term “idiom” to refer to a language using its common name, rather than using any of the international standards, such as “en”, “en-US”, or “en-CA-x-ca” for English and some of its variants.

Note
There is nothing to prevent you from using those abbreviations in your application!