Locale Names and LCID Deprecation
In the modern and managed worlds developers have been using Locale Names (a.k.a., IETF language tags, locale tags, language tags, culture tags, culture codes, BCP 47 tags, ll-cc, “en-US”, et cetera) for some time, but in other contexts LCIDs continue to be used. For those developers still using LCIDs, it is recommended to move to local names as they are more robust than LCIDs and can support many more markets.
What is a Locale?
A locale is a collection of features of the user’s environment that is dependent on language, country/region, and cultural conventions. The locale determines conventions such as sort order; keyboard layout; and date, time, number and currency formats.
What is an LCID?
Up until Windows Vista, each locale supported by Windows had a unique LCID identifier, a 32-bit value that consists of a language identifier and a sort order identifier. The locale identifier (LCID) was a Microsoft-proprietary numeric identifier for the locale, and before Windows Vista the LCID was used to identify installed operating system-defined locales. The NLS APIs in Windows support these predefined LCIDs.
As many applications do not care about the differences between sort identifiers, many developers see the LCID as synonomous with the lower 16 bit Language Identifier portion.
The Deprecation of LCIDs
LCIDS no longer provide a unique identifier for the locales recognized on Windows. With the restricted space of the LCID it is impossible to provide a unique identifier for each of the millions of language and region "locale" combinations that can be identified.
The deprecation of LCIDs began with Windows Vista, when APIs were added to recognize BCP 47 tags. Windows Vista also introduced custom locales, and those locales may not have an assigned LCID. Requests for LCIDs for those locales would use the LOCALE_CUSTOM_DEFAULT or LOCALE_CUSTOM_UNSPECIFIED identifiers. LOCALE_CUSTOM_DEFAULT if they locale happens to be the user default locale, otherwise LOCALE_CUSTOM_UNSPECIFIED.
Prior to Windows 8, the Windows team would assign new LCID values for each new locale added to Windows. However, starting with Windows 8, Windows started adding new locales that do not have a unique pre-assigned LCID value to identify them. Those locales act much like the custom locales introduced in Vista, typically using LOCALE_CUSTOM_UNSPECIFIED.
Windows 8 also introduced the concept of a "transient LCID" for the LCID-challenged "no-LCID" locales that do not happen to have a preassigned LCID. For some locales, Windows 8 will attempt to generate a "transient LCID" from a pool of reserved LCIDs. Transient LCIDs are just there to satisfy the requirement that the API uses an LCID, but the actual Transient LCID value you will get at runtime is essentially meaningless. If you do anything with a Transient LCID, it will very likely cause data loss. There are only a handful of values reserved for use as Transient LCIDs, and there are far more Transient Locales than there are Transient LCIDs. The Transient LCIDs are handed out on a first-come-first-served basis when a user adds a Transient Locale to their Windows Language Profile. Because the Transient LCID could be re-assigned at any time (such as when the user changes their Language Profile), and because it can mean a different Locale for a different user and/or on a different system, that is why such LCIDs are called Transient.
In Windows 10, Windows NLS expanded upon the transient concept to be able to recognize any well formed BCP-47 locale name presented to it. Windows will then "construct" a locale from the best information available. Those all share the LOCALE_CUSTOM_UNSPECIFIED LCID, unless they happen to be transient and get a random LCID assigned to them. With Windows 10, over half of the in-box locales are "no-LCID" locales, and many millions more are possible with the generous constructed locale capability.