Create a preview mode for non-default languages

Overview

Currently, you cannot make translations through the UI until a number of issues are resolved

But we met with @lauriii and we generally know a couple of things

1. For MVP: Translation will not happen in the Canvas UI. It will happen in content_translation/config_translation and/or TMGMT and its sub-modules
2. We will need a method to preview non-default languages in the UI

Proposed resolution

What we will need

1. A language dropdown
   
   This design is from Asymmetric translation but the dropdown should be similar

   It should show all languages on the site. These can be found here admin/config/regional/language. It should not just show languages the current entity is translated for. This is for 2 reasons. 1) Regions do not have their own URL, so you need to see regions in a different language even if no page/template is in that language. 2) A page/template can look different in another language even if it falls back to the default language because the entities it references might have translations, and templates will display field values in that language even if there is no template translation

   The dropdown should appear and be enabled when either config_translation or content_translation is enabled.

   This is true for Pages and ContentTemplates. For Pages, even though they are content entities, if only config_translation is on, regions could be translated. For ContentTemplates, even though they are config entities, even if only content_translation is enabled, the preview nodes could be translated.

2. Once the user selects a different language, it should open a view similar to the view you see when you click "Preview". We can't use the current Preview mode because that just displays the current layout HTML. (I, tedbow, am not a front end developer, so I am not sure how we should do this)
3. The previewing of the non-default language can use the existing layout endpoint at /canvas/api/v0/layout/{entity_type}/{entity} but with a language prefix fr/canvas/api/v0/layout/{entity_type}/{entity}

   \Drupal\Tests\canvas\Functional\TranslationTest::testTranslation proves that you can request a translation of the layout; see the assertions

   self::assertSame($expected_original_label, $get_name_in_api_response('/canvas/api/v0/layout/node/1'));
   self::assertSame("Drupal, c'est magnifique !", $get_name_in_api_response('/fr/canvas/api/v0/layout/node/1'));
   

Currently we can't make translations via the UI. To get around this:

1. For manual testing, update canvas_dev_translation to create translations on install for testing.

   Create an entity and translation for each type: Page, Template, Content Template.

   See \Drupal\Tests\canvas\Functional\TranslationTest::createCanvasNodeWithTranslation; this creates a node, but creating a Page should be the same.

   For ContentTemplate's we don't have an example in 1.x, so you will have to look at this https://git.drupalcode.org/project/canvas/-/merge_requests/898?file=8cd1...

   which is this MR https://git.drupalcode.org/project/canvas/-/merge_requests/898

   We don't have an example for PageRegion, but it should be similar to ContentTemplate (if this is a blocker, just skip PageRegion for now to get the issue moving)

2. The minimum canvas_dev_translation should create:
   • A canvas page with a header component and different text for the header in a translation
   • A PageRegion with a header component and different text for the header in the config language override
   • A ContentTemplate with a header component and different text for the header in the config language override
     and a header component that is linked to the node title

     There are a couple of other scenarios we can't test, but we should put @todo's to test later
     #3552924: When rendering content templates' linked prop sources (entity fields, host entity URLs …), load the relevant translation instead of the default
     #3583379: Referenced entities not resolved to current language translation in Evaluator
     (note to self, check if these issues are actually duplicates)

3. A playwright test can enable canvas_dev_translation and test with the entities and the translations it makes.

User interface changes

When viewing a page/template and either config_translation or content translation is enabled and canvas_dev_translation is enabled:

1. There should be a new dropdown to select a language to preview the entity in another language. It should show all languages enabled on the site; see /admin/config/regional/language
2. When a language is selected, the user is shown the current layout in a modal similar to if they clicked "Preview" in the toolbar now