Force language options to Gitalk comment system

[1v9]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines.
• Tests for the changes was maked (for bug fixes / features).
  • Muse | Mist have been tested.
  • Pisces | Gemini have been tested.
• Docs in NexT website have been added / updated (for new features).

PR Type

What kind of change does this PR introduce?

• Bugfix.
• Feature.
• Code style update (formatting, local variables).
• Refactoring (no functional changes, no api changes).
• Build related changes.
• CI related changes.
• Documentation content changes.
• Other... Please describe:

What is the current behavior?

Issue resolved: #651

What is the new behavior?

• Language can be foced.
• leave language empty, same as before
  
• Add es-ES
  

How to use?

In NexT _config.yml:

gitalk:
  enable: false
  github_id:  # Github repo owner
  repo:  # Repository name to store issues
  client_id:  # Github Application Client ID
  client_secret:  # Github Application Client Secret
  admin_user:  # GitHub repo owner and collaborators, only these guys can initialize github issues
  distraction_free_mode: true # Facebook-like distraction free mode
  # Gitalk's display language depends on user's browser or system environment
  # If you want everyone visiting your site to see a uniform language, you can set a force language value
  # Available value: en, es-ES, fr, ru, zh-CN, zh-TW
+ language:

Does this PR introduce a breaking change?

• Yes.
• No.

[ivan-nginx]

@1v9, @stevenjoezhang don't forget to set labels.