Purpose of this commit is to add tests on translations when using templates
and composer, on both email content (body, subject) and email layouting
content (view content, groups-based buttons, ...). This better highlights
some missing part of translation support, notably
* when being in mass mode: as content is taken from the composer and rendered
dynamically, it does not take the original translation from the template
even if it matches template content exactly;
* access button title is not translated as it is added during a separate
part of the process;
We also improve tests for ``mail.composer.mixin``. The wizard used in test
(mail.test.composer.mixin) is improve to behave more like a real invite
wizard: it runs on records (mail.test.composer.source) which are records
on which dynamic content is rendered. This highlights one issue with
translations: translations are fetched on current model (aka: composer
body and subject fields). However when using templates, translations are
stored on template model (aka: mail.template). This will be improved soon
by trying to fetch template translations when composer content matches
content from template.
Task-3046371 (Mail: Improve language propagation in mail and layouting)
Part-of: odoo/odoo#106072