Hello Annamaria,
If you have added the line
$string ['mynewtext1'] = 'hello world';
in the mod/certificate/lang/en/certificate.php file
Then in you certificate the line should be
certificate_print_text ($pdf, $x, $y + 15, 'C', 'Helvetica','', 10, get_string ('mynewtext1', 'certificate'));
Also there is no space between the $ signs and the following letter.