Summary of Changes

Multilingual site
When using a Create Article menu item using a specific category, whatever the language set for that category, the default language field is always set to the site language and can also be modified, which can cause unwanted errors.
In a multilingual site, an article tagged to a specific language should only be in a category tagged to the same language or All languages.

1. The first correction comes from #6966 where the intention was, when creating an article in backend to propose in the create article form by default the same language as the language filter articles manager was set to . Thanks to @Bakual for helping me finding this :)
   As this was not limited to Administration, it was forcing as default the site default language when creating an article in frontend.

This is now corrected here by limiting it to admin

We never remarked that overseen aspect because we had already a way in frontend to do that in the HtmlView.php

2. To prevent a user a to submit an article with the wrong language when a specific category is selected in the menu item, we then first have to check if this parameter is set in the view.
   If it is, then we should not propose as default the site language (which can be modifed) but instead code the FormModel preprocessForm() method to check the language of the category and, if not set to All languages, force the article to be set to the same language and read-only.

Note: The patch not only prevents error when the Create Article menu item deals with a specific category set to the same language as the site language, but also, cherry on the cake, let's create such menu items with a category set to another language than the site language. Both read-only.

For example:
One can have a Create Article menu item tagged to French and thus displayed in French frontend, with an English tagged specific category. The language field will be rightfully set as English per default and read-only.
As when one can do the more, one can do the less, let's test it.

Testing Instructions

Multilingual site with 2 languages.
Let's do the cherry on the cake :)
Create a Create Article menu item tagged to French with a specific category tagged to English in a French tagged menu.
Set access to Special.
Login frontend and click on the menu item.
Check the category and the language before submitting.
Check the Articles Manager for the new article.

Expected result AFTER applying this Pull Request

Creating the menu item

Frontend menu item displayed

Article form showing category

Article form showing read-only language field

Articles Manager showing newly created article

Note

One can have multiple Create Article menu items set to use different specific category (See my screenshot above).
Beware: once there is (are) such Create Article menu item(s), there should never be also a Create Article menu item NOT set to use a specific category.