Default on current environment language when there is no customer set on
the current coupon.
opw-2339815
note: part of this forward-port was not necessary because it was done in
98678199a3
X-original-commit: odoo/enterprise@d00981fe8f
closes #58473
closes odoo/odoo#58734
X-original-commit: f1b9ebe57f37bcfb271e00a7e9ba15a45493bd73
Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>