Check-in API: Return order locale, and allow to use order locale for …

[raphaelm]

…reasons

[luelista]

(+test case)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.46%. Comparing base (d928adf) to head (9c79bf5).
Report is 10 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4969   +/-   ##
=======================================
  Coverage   78.46%   78.46%           
=======================================
  Files         445      445           
  Lines       63290    63297    +7     
=======================================
+ Hits        49659    49666    +7     
  Misses      13631    13631           

Files with missing lines	Coverage Δ
src/pretix/api/serializers/checkin.py	88.70% <100.00%> (+0.18%)	⬆️
src/pretix/api/serializers/order.py	92.88% <100.00%> (+<0.01%)	⬆️
src/pretix/api/views/checkin.py	90.76% <100.00%> (+0.07%)	⬆️

... and 2 files with indirect coverage changes

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