Improve language negotiation

[gcotelli]

Improve request handlers to set up automatically the CurrentLocale when processing the Accept-Language headers, allowing easy translation of API responses.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.58%. Comparing base (d4eef66) to head (492ed48).
Report is 4 commits behind head on release-candidate.

Additional details and impacted files

@@                  Coverage Diff                  @@
##           release-candidate     #196      +/-   ##
=====================================================
- Coverage              99.61%   99.58%   -0.03%     
=====================================================
  Files                    166      166              
  Lines                  13457    13631     +174     
=====================================================
+ Hits                   13405    13575     +170     
- Misses                    52       56       +4     

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