Language class should use property 'label' to be consistent with entities

Problem/Motivation

Although Language is not an entity, it almost shares a structure with ConfigurableLanguage and that uses label (just as other entities do).

Beta phase evaluation

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Postponed for 8.1.x because it does not meet the qualification on https://www.drupal.org/core/beta-changes → ?

Proposed resolution

• In core/lib/Drupal/Core/Language/Language.php change name to label to be consistent.
• Update all uses.
• Keep old function names as a BC wrapper.
• Need a follow up issue postponed until 8.1.x. to remove the deprecated methods.

Remaining tasks

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• git instructions for creating patch | Contributor task documentation for creating a patch
• perform patch reroll
• Review patch to check it fixes the issue, the change is properly documented and for coding standards. Make sure patch stays within scope of just this issue. | Contributor task documentaiton for reviewing patch
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• Update the issue summary noting if allowed during the beta Instructions →
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User interface changes

No

API changes

No

Related issues

• Split off from #2239497: [Meta] Fix ConfigurableLanguageManager getLanguages() →
• ?