Allow preferred language selection for link field

I think this needs a change notice. The code looks very good though.

This worked perfectly for our use case. We have a language selection modal which leverages the link field type to link to internal references in different languages. Thanks for this.

EDIT: Should specify, the patch from #7 was RTBCd.

Tagging to make sure that everyone knows what there is still to do to get this in.

Re-rolled the #11 patch against 8.6.x branch so marking this issue for "Needs Review".

Any chance we could call the flag whether specifying language is allowed something other than just "language"? We also have the actual specified language named "language" and it gets confusing. Could we call the flag "specify_language" or "set_language" or "allow_set_language" or someone has a better idea?

So, I feel like there needs to be a change here. I feel like there needs to be another option for "Use page language", and if that option is set then the language option is not sent along to the formatter (reverting the formatting to previous functionality where the language option was never passed to the URL builder).

Reason being, when this patch is applied, and the field settings are updated to "Optional", when re-saving old content 1 of several things can happen:

1) A language is picked and saved in the field's options.
2) Not specified (und) is written to options.
3) Not applicable (zxx) is written to options.

When you're running a site where ALL of the aliases have an assigned language this causes a problem because the language option is now always sent to the URL builder, which inevitably gets passed to the alias manager to get the alias for the path and the alias manager looks at one of two things when determining what alias to pick:

1) If there is a langcode passed, use that langcode.
2) Otherwise, get default language.

As soon as you save a field with this patch applied, a langcode will always be passed to the alias manager and your field will never render in the language of the page again.

The counter argument to this is, if you're editing the content you know what language you're editing the content in, so just set the language of the link as the same version of the language you're editing. Okay, but say you have hundreds of links that now have this option enabled for it, you are now asking the site builder to remember to set the link's preferred language to the language you're currently editing every single time. That's a lot to ask in my opinion.

I think the best way to make this backwards compatible with existing sites is to add a preferred language option of "Page language" or "Current language", something akin to that, and make that the default choice for the preferred language property. That will save people the headache of saving their content and all of a sudden their links on pages stop showing the alias of the page and show "node/325" instead.

In any case, if anyone needs clarification on what I'm trying to say, would love to clear anything up. It's the end of the day and I'm a little spent!

EDIT 1: One more point, I think this only needs to be done if the preferred language is set as optional. If it's required then it's required, no real argument there, you MUST pick a language. If it's optional though you should be able to ignore this setting altogether. If the property is disabled, then if there already exists a language option in the field's storage, the language option should be removed from storage.

Patch for signaling to use current page's language, which actually defaults to removing the language option stored with the field. Effectively it reproduces the previous behavior.

This needs more work. It should ONLY be available if the link field is configured to optionally select preferred language. In addition, when the use page language checkbox is displayed it would be nice to leverage state api to hide the preferred language select.

Applies to 8.9.x and explicitly checks for language being a valid index value, otherwise you get a notice which is breaking AJAX interactions on paragraphs.

Sometimes the language option for a link field is an empty array, which evaluates to TRUE with isset.

Re-rolled #29 for Drupal 9.3

Here is a reroll for 9.3.2:

Reroll 9.3.2

Fixing deprecated code.

Fixing tests with the missing schema.

#38 patch is working when we use link option for link type on Manage form display.
But when we select

'link (with attributes)'

, we need to apply a patch in link_attributes module .

I have uploaded the patch with fixed cs error