Fix #11518: Application error #1904 has no error description

If an invalid signup captcha is entered upon signing up a new account,
application error 1904 is raised. However, the error description
accompanying the error ID of 1904 is not shown on the error page.

This problem is due to a bug within lang_get_default() from lang_api.php
where the language of a non-authenticated user is set to boolean 'false'
instead of the default language of the entire MantisBT installation.

core/lang_api.php

@@ -117,11 +117,11 @@ function lang_get_default() {
 	}
 
 	# Otherwise fall back to default
-	if( null === $t_lang ) {
+	if( !$t_lang ) {
 		$t_lang = config_get_global( 'default_language' );
 	}
 
-	if( 'auto' == $t_lang ) {
+	if( $t_lang == 'auto' ) {
 		$t_lang = lang_map_auto();
 	}