feat: Setting default language to english in APISIX Dashboard

[dileepajayakody]

Please answer these questions before submitting a pull request, or your PR will get closed.

Why submit this pull request?

• Bugfix
• New feature provided
• Improve performance
• Backport patches

What changes will this PR take into?

This PR sets the default language in apisix-dashboard to en-US.

Related issues

Checklist:

• Did you explain what problem does this PR solve? Or what new features have been added?
• Have you added corresponding test cases?
• Have you modified the corresponding document?
• Is this PR backward compatible? If it is not backward compatible, please discuss on the mailing list first

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.13%. Comparing base (ad7e3e7) to head (a87f457).
Report is 180 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2212      +/-   ##
==========================================
+ Coverage   65.16%   68.13%   +2.97%     
==========================================
  Files         184      127      -57     
  Lines        7257     3352    -3905     
  Branches      824      824              
==========================================
- Hits         4729     2284    -2445     
+ Misses       2244     1068    -1176     
+ Partials      284        0     -284     

Flag	Coverage Δ
backend-e2e-test	?
backend-unit-test	?
frontend-e2e-test	68.13% <ø> (ø)

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[bzp2010]

LGTM