Fix multilingual preferred labels in CSV exports

[jvendetti]

Summary

• Keep CSV Preferred Label as a single string column
• Prefer English labels, including BCP-47 English variants, then untagged labels
• Add regression coverage for multilingual label selection and legacy prefLabel fallback

Fixes #288

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.28%. Comparing base (74da285) to head (7c7427f).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #292      +/-   ##
===========================================
+ Coverage    80.21%   80.28%   +0.06%     
===========================================
  Files          100      100              
  Lines         6753     6781      +28     
===========================================
+ Hits          5417     5444      +27     
- Misses        1336     1337       +1     

Flag	Coverage Δ
ag	80.20% <100.00%> (+0.06%)	⬆️
fs	80.26% <100.00%> (+0.08%)	⬆️
gd	80.22% <100.00%> (+0.06%)	⬆️
unittests	80.28% <100.00%> (+0.06%)	⬆️
vo	80.20% <100.00%> (+0.08%)	⬆️

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