Change the search URL

From WPEngineer (with edits):

Add the following to functions.php in your theme:

function fb_change_search_url_rewrite() {
    if ( is_search() && ! empty( $_GET['s'] ) ) {
        wp_redirect( home_url( "/busca/?s=" ) . urlencode( get_query_var( 's' ) ) );
        exit();
    }   
}
add_action( 'template_redirect', 'fb_change_search_url_rewrite' );

Add the following to your htaccess file:

RewriteCond %{QUERY_STRING} ^s=(.*)$ [NC]
RewriteRule ^$ /busca/%1 [NC,QSA,L]

Note: Code is untested

Edit: Minor modifications made to answer the question more thoroughly