For admin panel only!
This contribution will display the english language file if the language file for the selected display language is missing.
Why not implement this feature in catalog?
osCommerce catalog is only including the system language file from application_top. The catalog language files are set in each document such as address_book.php, index.php, etc and will require way too many files to edit. There is no way to load a secondary language file since the scripts will still halt if the primary language file is missing due to the function require(languagefile).