In PHP 5.3, mb_language() returns "neutral" by default for me.
(PHP 4 >= 4.0.6, PHP 5)
mb_language — Set/Get current language
Set/Get the current language.
Used for encoding e-mail messages. Valid languages are "Japanese", "ja","English","en" and "uni" (UTF-8). mb_send_mail() uses this setting to encode e-mail.
Language and its setting is ISO-2022-JP/Base64 for Japanese, UTF-8/Base64 for uni, ISO-8859-1/quoted printable for English.
If language is set and language is valid, it returns TRUE. Otherwise, it returns FALSE. When language is omitted, it returns the language name as a string. If no language is set previously, it then returns FALSE.