Fix Gittalk language wrong for default value, the == '' failed

[dreampiggy]

PR Checklist

• [x] The commit message follows guidelines for NexT.
• [x] 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 features).

PR Type

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

What is the current behavior?

The default _config.yml using Gittalk will not use the default window.navigator.language for language. Not designed behavior

gitalk:
  language:

Issue resolved: N/A

What is the new behavior?

• Screenshots with this changes: N/A
• Link to demo site with this changes: N/A

How to use?

In NexT _config.yml:

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