If you would like to create a new language file you can do so by translating one already created or by downloading one that has already been translated to your language.
To create your own language file it is recommended to start with the english.php file. If you open the file in a Text Editor you will notice it looks like this:
define("LANG_YES", "Yes"); define("LANG_NO", "No"); define("LANG_ERROR", "Error");
You would then change the portion in the second parentheses. Example:
define("LANG_YES", "sí");
Which then would display sí instead of yes.