Fix i18n locale activation

[VladislavLysykh]

Added tests for i18n locale activation.

Changes:

• added coverage for locale activation
• verified translation loading
• verified locale switching behavior

Test results:
703 passed, 69 skipped

[codspeed-hq]

Merging this PR will not alter performance

✅ 15 untouched benchmarks

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_{Comparing VladislavLysykh:main (76964bc) with main (976484a)}

[hugovk]

How is this testing locale activation? It's only testing some noop functions that don't really do anything.

[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 99.55%. Comparing base (976484a) to head (76964bc).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
tests/test_i18n.py	87.50%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #327   +/-   ##
=======================================
  Coverage   99.55%   99.55%           
=======================================
  Files          12       12           
  Lines         900      908    +8     
=======================================
+ Hits          896      904    +8     
  Misses          4        4           

Flag	Coverage Δ
macos-latest	97.57% <87.50%> (+0.02%)	⬆️
ubuntu-latest	97.57% <87.50%> (+0.02%)	⬆️
windows-latest	95.70% <87.50%> (+0.03%)	⬆️

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