OK. You are using a .htaccess rule.
If you don´t use an .htacces rule you will obtain a url like this:
http://example.com/index.phpen
and you should modify the library to place a ‘/’ between index.php and language uri.
Can I suggest that this little advice could be added to wiki?
But now I get this error message:
An Error Was Encountered
Unable to load the requested language file: language/spanish_lang.php
or english,etc..