Reimplement usage of Google Translate API as an adapter registration, to be able to provide different services

[erral]

Addresses #467

Motivation

The current implementation requires anyone wanting to provide some other external translation service, to override the existing view and do the logic there.

I think that the implementation proposed here is flexible enough to be able to register several different services

Introduced components

A new interface IExternalTranslationService is introduced which has several attributes and methods:

• is_available(): to return whether the adapter is available (for instance, is the Google translation API key entered into the control panel?)
• available_languages(): a list of language pairs (source, target), that this service supports.
• order: the order in which this adapter will be executed. This way, one can prioritize some services over others.
• translate_content(content, source, target): the actual function that does the translation call.

I have converted the function that currently calls Google Translate into an adapter providing IExternalTranslationService

Strategy

The current browser view that does the call to Google Translate has been rewritten to get all registered adapters that are available.

Then it checks that the adapter supports the language-pair (source, target) translation.

If so, it requests the translation and returns it.

Other solutions I thought of

I thought also on creating a ZCML tag just for registering this kind of adapters, but found easier going the adapter way.

Documentation

If this is accepted, a documentation PR will be issued from this branch, where this adapter registration has been documented:

Branch: https://github.com/plone/documentation/tree/erral-adapter-registration
Documentation of the adapter: https://github.com/plone/documentation/blob/erral-adapter-registration/docs/i18n-l10n/use-an-external-translation-service.md#using-other-translation-services

Other

I will update the PR with the changes in #462 if/when merged

[mister-roboto]

@erral thanks for creating this Pull Request and helping to improve Plone!

TL;DR: Finish pushing changes, pass all other checks, then paste a comment:

@jenkins-plone-org please run jobs

To ensure that these changes do not break other parts of Plone, the Plone test suite matrix needs to pass, but it takes 30-60 min. Other CI checks are usually much faster and the Plone Jenkins resources are limited, so when done pushing changes and all other checks pass either start all Jenkins PR jobs yourself, or simply add the comment above in this PR to start all the jobs automatically.

Happy hacking!

[erral]

@jenkins-plone-org please run jobs

[erral]

@jenkins-plone-org please run jobs

[erral]

@jenkins-plone-org please run jobs