Unlisted API change: langcode

[VasasA]

Unlisted API change in change records:

• "Node" entity has a langcode property instead of language property.
• If the module uses the language key object within the $message array parameter in hook_mail(), now has a langcode property instead of language property.

[klonos]

Thanks @VasasA 👍 ...this is related to #3621

[laryn]

Related thing I noticed today: the API examples for language are wrong. 😳

https://github.com/backdrop/backdrop/blob/b8a1bca86ac6463d8a6f7556aa20073cfd543e43/core/modules/system/language.api.php#L29

switch ($language->language) {

(and similar) should be:

switch ($language->langcode) {