RATIONALE
Naming conventions
* content = core message / subject (+ other fields);
* email layout = layouting applied to messages when sent to recipients who
receive notifications by email e.g. access button, ...;
* comment mode / mailing mode: composer main composition mode: posting on
record(s) / sending a mailing on records;
Content situation when using a template to post on a composer
* comment, monorecord mode: choose a template, content is rendered directly
from template side according to 'lang' field definition -> ok;
* comment, multirecord mode / mailing mode: choose a template, content is
the raw content. At post / send, content is rendered and translated if
matching template content -> ok;
Email layout situation when posting / mailing
* comment: layout translated based on template 'lang' field definition
or fallback on current user's lang;
* mailing: no layout supported;
PURPOSE
Translate layout into recipient's lang when available, instead of either
current user or template defined lang. As it is contextual better translate
it to the end user lang.
SPECIFICATIONS
Move recipients and email layout rendering into a method returning an iterator.
That way a list of recipient groups (as given by '_notify_get_recipients') can
be split into sub-groups, with each having its own rendering values for
email notifications.
Do a split / recipient lang. It is already fetched when notifying messages
as part of '_get_recipient_data' returned values. We now return recipients
groups per lang and per usage type (user, portal, customer, ...). Rendering
values are computed for each group as several values depend on the lang (
actions, notification buttons, global layout, ...).
Layouts when posting are dependent on recipient lang, and not on current user
anymore. Template lang that forced the content lang is now used only as a
fallback when recipient have no lang.
SUMMARY
When a user in Spanish, posts using a template specifying the lang of the
customer on a record whose customer is in French, with a follower being
in English
* content will be translated following template choice, aka french;
* layouting for the french customer will be in french, layouting for the
english customer is in english, while the core content is always the
same and in french;
When a user in Spanish, posts some custome content on a record whose customer
is in French, with a follower being in English
* content is whatever the user wrote;
* layouting for the french customer will be in french, layouting for the
english customer is in english, while the core content is always the
same and in french;
Task-3046371 (Mail: Better Language Support in Composer)
Task-2555155 (Mail: Translate notification action/access buttons)
Task-3186426 (Mail: Support notification layout in template / email composer)
Part-of: odoo/odoo#106177
Co-authored-by: Julien Banken <jbn@odoo.com>
