Lang selector show disable lang when creating new content

I've been kinda busy lately, but i'm happy to review and accept a patch.

If I find some time i'll look into it.

Worked a bit on this issue on drupaljam, but was not able to finish this.

In my opinion we'll need a custom permission for this, because in some cases the module can be used to disable a language where the content is being created, and in that case you'll need to be able to create content in a disabled language.

Done:
- Created custom permissions
- Started on writing a custom widget alter with permisson check

TODO:
- Disable the languages in the edit form.

Notes:
I tried with a form alter and field_widget_alter, but it seems that it's not a standard select list, but something special for the languages, so I don't know if you can easily remove items from the select list

There is a way to alter language_select widget options, but as there's lot of processing going on, those options are available bit late,
dealing with it in #after_build callback does the job

Hello sandboxpl i tested your patch in #5 and it seems to work well. Another thing to consider could be: When on a node edit form you click the 'translate' tab. All languages are still visible including the disabled ones, maybe these should be hidden? The 'add' link does redirect however.

Patch #5 doesn't work for me. I see the permission, but the select widget continues to display all languages even for people without the permission.

UPDATE:
Patch #5 works for me on a new test environment a created locally.

@michaelvanh altering content translation tables is a tricky thing, I think it should be reviewed and executed in a separate issue, I've created new one for that: https://www.drupal.org/project/disable_language/issues/3091554

reviewed and committed do dev,
thanks for providing code and testing this one !