[WordPress] Modify default (not translated) language strings of a plugin. Beware: Might result in performance issues when applied to many strings.

wp-modify-text-strings.php

<?php 
/**
 * Translate text strings in WordPress programmatically.
 *
 * Assuming plugin intorduces "Foo_Plugin" class, has text domain "foo-plugin".
 * 
 * @param mixed $translated
 * @param mixed $original
 * @param mixed $domain
 * @return string
 */
function gp1410201916( $translated, $original, $domain ) {

    /* Only strings from given plugin */
	if( ! class_exists( 'Foo_Plugin' ) || 'foo-plugin' !== $domain ) {
		
		return $translated;
	}
	
	/* 'Original string' => 'New string' */
	$strings = array( 'Wordpress' => 'WordPress' );
	
	if( isset( $strings[ $original ] ) ) {
		
		$translations = &get_translations_for_domain( $domain );
		$translated   = $translations->translate( $strings[$original] );
	}

	return $translated;
}
add_filter( 'gettext', 'gp1410201916', 10, 3 );