Improve warning upon adding the Language plugin

FYI: this approach was implemented after very careful consideration in #3273983: Do not assume that plugin supporting <tag attr> also supports <tag> in SourceEditingRedundantTags and upgrade path.

You suggest:

Better yet, just tell them to add <span lang dir> to Source Editing once #3388698: Adding the language plug-in does not add the correct span tag attributes is fixed.

But the message literally says:

[…] Enable a plugin that supports creating this tag. If none exists, you can configure the Source Editing plugin to support it.

… which is what you're asking for! (⚠️ With one important exception: you were trying to add <span lang dir>, you should have only added <span>, per the message!)

I suspect you just find the current message not precise enough? I can see that!

Would this be better?

[…] Enable a plugin that supports creating this tag. If none exists, you can configure the Source Editing plugin to support it: add to the list of "Manually editable HTML tags".

Would that help?

I'd really prefer Drupal to just tell them to manually add the tag and not send people on a hunt to find a plug-in which may or may not exist when I can solve it with a click on the Source Editing tab and type 6 characters.

That's not recommended from a CKEditor 5 POV. But … you're right that this is the only pragmatic choice!

So let's change this!

Could you roll a patch or create a merge request with your proposed text? 😊

I'll take this.

The fundamental issue behind this is that the ckeditor5_language plugin requires that you are able to *edit* span tags, but does not guarantee that you can create them. Only the "Source Editing" plugin allows this. It's not always desirable to allow users to edit the source even when they can set language tags.

Is it reasonable to update ckeditor5.ckeditor5.yml to read like this?

ckeditor5_language:
  ckeditor5:
    plugins: [language.TextPartLanguage]
  drupal:
    label: Language
    library: ckeditor5/internal.ckeditor5.language
    admin_library: ckeditor5/internal.admin.language
    class: Drupal\ckeditor5\Plugin\CKEditor5Plugin\Language
    toolbar_items:
      textPartLanguage:
        label: Language
    elements:
      - <span>
      - <span lang dir>

Changes would also have to be made to the relevant tests, notably Drupal\Tests\ckeditor5\FunctionalJavascript\AdminUITest and Drupal\Tests\ckeditor5\FunctionalJavascript\CKEditor5Test

See also https://www.drupal.org/node/3283526#comment-15391076

Just want to note here that today I tried turning off manually edit source HTML option because of a bug where I get an error that suggests there is an option that is already covered by a plugin, but lists no tags in the error. I think this is related to this issue https://www.drupal.org/project/drupal/issues/3410100 which does have a patch. I didn't apply that patch because I was okay with taking away the option for people to manually edit source, which I did. That is when I ran into the error described in this issue. And I want to express how unfortunate it feels to see the recommendation here is to re-enable the option to manually edit source HTML, which is broken. At least there is a patch, but thought it was worth mentioning that there are issues right behind the fix that has been recommended throughout this thread.