- Test 1 (AllEnKeys_HaveNonEmptyFrTranslation): verifies every EN key has a non-empty, non-bracketed FR translation
- Test 2 (FrStrings_ContainExpectedDiacritics): spot-checks 5 keys for correct diacritics (é/è)
- Both tests pass — FR file already contains correct diacritics
- Instance singleton test
- EN string lookup test
- FR culture fallback test
- Missing key returns bracketed key
- PropertyChanged fires with empty string on culture switch
- Same culture does not fire PropertyChanged