RtlIsValidLocaleName - NtDoc

Native API online documentation, based on the System Informer (formerly Process Hacker) phnt headers
#ifndef _NTRTL_H
// Locale
#if (PHNT_VERSION >= PHNT_VISTA)

// private
NTSYSAPI
BOOLEAN
NTAPI
RtlIsValidLocaleName(
    _In_ PCWSTR LocaleName,
    _In_ ULONG Flags
    );

#endif
#endif

View code on GitHub

This function is documented in Windows SDK.