[FIX] payment: use a better fallback when no partner language

In case the partner has no language (not a required field), the fallback on the
payment transaction was 'en_US'
en_US may however be an invalid choice for partner_lang field as it is
a selection field that only has the activated languages (result of the call to
self.env['res.lang'].get_installed())
A database created in a different language does not have en_US activated

Fallback on user language instead of False as the partner_lang is expected by
some payment providers and may produce an error is empty

Fixes odoo/odoo#33750

closes odoo/odoo#33870

Signed-off-by: Martin Trigaux (mat) <mat@odoo.com>

addons/payment/models/payment_acquirer.py

@@ -879,7 +879,7 @@ class PaymentTransaction(models.Model):
 
             values.update({
                 'partner_name': partner.name,
-                'partner_lang': partner.lang or 'en_US',
+                'partner_lang': partner.lang or self.env.user.lang,
                 'partner_email': partner.email,
                 'partner_zip': partner.zip,
                 'partner_address': _partner_format_address(partner.street or '', partner.street2 or ''),