change default locale from en_EN to en_US

In propel 1.6.8 the default locale was changed from en_EN to en_US because en_EN seems to not be a standard locale : https://github.com/propelorm/Propel/commit/66d374071d0a37c8ba51f41ada3c8291fe4fb1ba#generator/lib/behavior/i18n/I18nBehavior.php

In propel 2 the default locale is again en_EN, in this PR I change the default locale to en_US.

Thanks