It's because it's using Helvetica font which doesn't have unicode characters - if you can use Deja Vu Sans or another font that has unicode it should work.
IGNORE rest of answer below. As per comments Cesar found that under Administer - Communications - Label Formats you can change the font in there.
You can try to hack CRM/Utils/PDF/Label.php to hardcode the font: https://github.com/civicrm/civicrm-core/blob/5.20.3/CRM/Utils/PDF/Label.php#L180
I'm not sure the right spelling to get the font. If you look in vendor/tecnickcom/tcpdf/fonts/dejavusans.php it seems like it might be
DejaVuSans, without spaces.
You can also try this instead:
- Copy vendor/tecnickcom/tcpdf/config/tcpdf_config.php somewhere, e.g.
- Edit the line inside it for PDF_FONT_NAME_MAIN so it reads
define('PDF_FONT_NAME_MAIN', 'dejavusans'); (again I'm not sure of the exact spelling).
- In civicrm.settings.php add these lines: