where to set the PO file for a plugin?

Question

I have a Cimy user plugin and I want to make translations and then force the Cimy user to read from my translation. Should I define the language in wp-config.php in the root folder?
I do not know where to change it?

I have define('WPLANG', 'fa_IR'); in wp-config.php and it is reading from path

\wp-content\languages

Should I change the fa_IR PO edit and just add the ones from Cimy user in the bottom of the WordPress fa_IR PO? or I should use separate file?