i18n support in Emails and Messages app

[krzysztofwolski]

Problem I would like to solve

Send fully translated emails to customers.

Intro

API

Saleor API provides translation models that the App could use. To choose a language, we can:

• Use customer preference from User model
• Use language code of the order (assuming storefront assigns it during the checkout)

Currently, the API does not provide a way to automatically detect language and add a proper translation to the subscription. It means additional API calls after receiving the webhook.

Templates

Sendgrid and SMTP providers lack dedicated i18n support and require conditional rendering with Handlebars to localize text. Sendgrid documentation with examples for reference.

For initial i18n implementation in the EAM, it would be recommended approach. As a potential improvement in the future, we could make configuration variants depending on the language:

• user creates a base configuration, it will be used as a fallback if there's no variant related to the email language
• user creates a dedicated variant for the language. A variant would provide an interface to choose a different sender, message subject and template.

When a webhook arrives, the App checks for any configuration variant of the event language. If none is found, the base configuration is used.

The proposed solution increases the app's complexity (both for implementation and for the user).

Impact

• modify webhook subscriptions to add customer/event language code
• add queries for model translations
• add translations to template payload

### Tasks
- [ ] https://github.com/saleor/apps/issues/727

[krzysztofwolski]

The #816 PR added translated fields for the product and variant names. Other fields, like attribute names, are not translated.