First, does the language specific option work?
Second, the default language is "C" which must exist in your lang/installed_languages.inc like:
/* How to make new entries here for non-packaged languages:
-- 'code' should match the name of the directory for the language under \lang
.-- 'name' is the name that will be displayed in the language selection list (in Users and Display Setup)
-- 'rtl' only needs to be set for right-to-left languages like Arabic and Hebrew
-- 'encoding' used in translation file
-- 'version' always set to '' for manually installed languages.
-- 'path' installation path related to FA root (e.g. 'lang/en_US').
$installed_languages = array (
'code' => 'C',
'name' => 'English',
'encoding' => 'iso-8859-1',
$dflt_lang = 'C';
Now we need to study the code as to where these routines in the said file are called and how the defaults get assumed.
The file locale.inc is "included" in the
1. function install_hooks() in the file includes/hooks.inc
2. function set_language($code) in the file includes/lang/language.php