The reference source for string.cs leads to a managed-native boundary in a call to comnlsinfo.cpp :: InternalCompareStringOrdinalIgnoreCase(), which I cannot find in public open source. There is "slow-path" case-insensitive comparison logic in there somewhere that handles non-ASCII characters gracefully though more slowly. Try to get a reference to this source code and translate the slow-path logic, then add tests for above-ASCII case-insensitive comparisons.
The reference source for string.cs leads to a managed-native boundary in a call to comnlsinfo.cpp :: InternalCompareStringOrdinalIgnoreCase(), which I cannot find in public open source. There is "slow-path" case-insensitive comparison logic in there somewhere that handles non-ASCII characters gracefully though more slowly. Try to get a reference to this source code and translate the slow-path logic, then add tests for above-ASCII case-insensitive comparisons.