Legal terms live under specific URL as their translations are not
managed in the same manner.
Layout is like this:
/terms/enterprise.html (EN)
/terms/i18n/enterprise_fr.html (FR)
This commit adapts the generation of alternative languages links for each
"legal terms page", so that it targets the correct i18n link, or goes
back to the canonical EN one, depending on the target language.
closesodoo/documentation#7383
X-original-commit: 8062e54a7e
Signed-off-by: Olivier Dony (odo) <odo@odoo.com>