where to set the PO file for a plugin?

First of all, the plugin should support translation.

If it is so, the directory where to put po/mo file is defined by plugin, usually they are in a directory /langs or /languages in plugin directory.

That directory is (should be) defined by plugin using load_plugin_textdomain

The plugin you are using has a subfolder cimy-user-extra-fields/langs where you can find the existing localization files.

Your files (cimy_uef-fa_IR.po / cimy_uef-fa_IR.mo), should be putted in the same folder.

To create these files you can use the software PO Edit a powerful software, maybe not very easy to use.

If you already don’t know it I suggest you the plugin Codestyling Localization a powerfull well written WP plugin that will help you trought the entire process of transalitng plugins, themes and wordpress itself.

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