i18nTranslateAdapterInterface

Makes the Zend_Translate_Adapter base class aware of file naming conventions within SilverStripe.

Needs to be implemented by all translators used through i18n::register_translator().

A bit of context: Zend is file extension agnostic by default, and simply uses the filenames to detect locales with the 'scan' option, passing all files to the used adapter. We support multiple formats in the same /lang/ folder, so need to be more selective about including files to avoid e.g. a YAML adapter trying to parse a PHP file.