Assuming the plugin makes use of localization at all, you could for example try exchanging the possible load_plugin_textdomain
call with load_textdomain
where you can point to the file you like.
A cleaner way would be using the plugin_locale
filter that let’s you modify the locale used by a plugin:
function my_plugin_locale_filter($locale, $domain)
{
if($domain === 'the_plugins_textdomain')
{
return 'cy_CY';
}
return $locale;
}
add_filter('plugin_locale', 'my_plugin_locale_filter', 10, 2);
That way the plugin can more or less safely be updated without the custom code being overwritten.