xhtml_style: don't allow google translate

Some users have been using google chrome builtin translation system with
ERP5 back office (xhtml style), but the way google translation changes
the DOM sometimes break ERP5 interface.
This also have data privacy issue as this translation is done on google
servers.

https://www.w3.org/International/questions/qa-translate-flag recommends
using translate=no but the approach of the meta tag might be preferable
for our case as it prevents sending the page content to translation
service.

/reviewed-on nexedi/erp5!879