Multilingual User Interface
Multilingual User Interface (MUI) is the name of a Microsoft technology for Microsoft Windows, Microsoft Office and other applications that allows for the installation of multiple interface languages on a single system. On a system with MUI, each user would be able to select their own preferred display language. MUI technology was introduced with Windows 2000 and has been used in every release since (up to Windows 10). The MUI technology was covered by an international patent titled "Multilingual User Interface for an Operating System". The inventors are Bjorn C. Rettig, Edward S. Miller, Gregory Wilson, and Shan Xu.

Functionally, MUI packs for a certain product perform the same task as localized versions of those product, but with some key technical differences. While both localized versions of software and MUI versions display menus and dialogs in the targeted language, only localized versions have translated file and folder names. A localized version of Windows translates the base operating system, as well as all included programs, including file and folder names, objects names, strings in registry, and any other internal strings used by Windows into a particular language. Localized versions of Windows support upgrading from a previous localized version and user interface resources are completely localized, which is not the case for MUI versions of a product. MUI versions of a product do not contain translated administrative functions such as registry entries and items in Microsoft Management Console. The advantage of using MUIs over localized versions is each user on a computer could use a different language MUI without requiring different versions of software installed and dealing with the conflicts that could arise as a result. For example, using MUI technology, any version of Windows can host Windows applications in any other language.

MUI in Windows 2000 and Windows XP
MUI products for these versions were available only through volume agreements from Microsoft. They were not available through retail channels. However, some OEMs distributed the product.

List of languages in Windows XP 
Up to Windows XP, MUI packs for a product are applied on top of an English version to provide a localized user experience. There are a total of 5 sets of MUI packs.

Set 1 

 German
 French
 Japanese
 Korean
 Chinese (Simplified)
 Chinese (Traditional)

Set 2 

 Arabic
 Hebrew
 Spanish
 Italian
 Swedish
 Dutch
 Portuguese (Brazil)

Set 3 

 Norwegian
 Danish
 Finnish
 Russian
 Czech

Set 4 

 Polish
 Hungarian
 Portuguese (Portugal)
 Turkish
 Greek

Set 5 

 Bulgarian
 Estonian
 Croatian
 Latvian
 Lithuanian
 Romanian
 Slovak
 Slovenian
 Thai

MUI in Windows Vista and Windows 7

Windows Vista 
Windows Vista further advanced MUI technology with support for single, language-neutral, language-independent binary files supporting multiple language skins, with the language-specific resources contained in separate binaries. The MUI architecture separates the language resources for the user interface from the binary code of the operating system. This separation makes it possible to change languages completely without changing the core binaries of Windows Vista, or to have multiple languages installed on the same computer while using the same core binaries. Languages are applied as language packs containing the resources required to localize part of or the entire user interface in Windows Vista.

MUI packs are available to Windows Vista Enterprise users and as an Ultimate Extras to Windows Vista Ultimate users.

Beginning with Windows Vista, the set of associated MUI APIs are also made available to developers for application development.

At launch, the following 16 language packs were released:

 Danish
 German
 English
 Spanish
 French
 Italian
 Dutch
 Norwegian
 Portuguese (Brazil)
 Finnish
 Swedish
 Russian
 Korean
 Chinese (Simplified)
 Chinese (Traditional)
 Japanese

On October 23, 2007, the remaining 19 language packs were released:

 Czech
 Estonian
 Croatian
 Latvian
 Lithuanian
 Hungarian
 Polish
 Portuguese (Portugal)
 Romanian
 Slovak
 Slovenian
 Serbian
 Turkish
 Greek
 Bulgarian
 Ukrainian
 Hebrew
 Arabic
 Thai

Windows 7 
MUI packs are also available to Windows 7 Enterprise and Ultimate edition users. Beginning with Windows 7, Microsoft started referring to MUIs as "Language Packs," although this isn't to be confused with Language Interface Packs (LIP).

MUI in Windows 8/8.1/RT and Windows 10

Beginning with Windows 8/RT, most editions of Windows are able to download and install all Language Packs, with a few exceptions:

 In Single Language editions of Windows, only one language pack is allowed to be installed,  the same behavior as editions of Windows 7 and earlier that are not Enterprise or Ultimate.
 In OEM editions of Windows, the exact language packs that are preinstalled/available for download depend on the device manufacturer and country/region of purchase (and the mobile operator for devices with cellular connectivity). This is a mixture of a local-market feature and a feature for everyone everywhere. There may be multiple display languages preinstalled on the device by the manufacturer and/or wireless carrier, but each manufacturer and/or wireless carrier installs two different sets of languages: one set of preloaded languages and one set of languages that can be installed by the end user. This rule is currently used in Windows Phones as of Windows Phone 7 and PCs as of Windows 8 (since Windows 8 and Windows Phone 8 share the same Windows NT kernel) and was later dropped in Windows 10 version 1803, but was later quietly reinstated as of Windows 10 version 1809. An end user could install a retail license on top of an OEM license by performing a clean install through the Media Creation Tool to circumvent the region locking and install any display language that they want.
The Windows update process does not affect the currently installed display languages in any way, but it may give the end user access to newly released language packs made available by the OEM (PCs only). However, when installing a new feature update, it may change the display language back to the one set during the initial setup process. For example, if the Samsung ATIV Smart PC on AT&T is upgraded from Windows 8.1 to Windows 10 Anniversary Update (not necessarily done in one go), it will now be able to install Portuguese (Brazil), Vietnamese, Chinese (Simplified and Traditional), and Japanese in addition to English, Spanish, French, German, Italian, and Korean (the last three languages can be downloaded by the end user at the time of its launch), just like with the Galaxy S8 series and the Verizon-based Galaxy Book.
On the other hand, a Samsung Galaxy Book device does not support Afrikaans as a display language, because Samsung apps do not officially support Afrikaans. Furthermore, cellular variants of the Galaxy Book laptops sold in North America support fewer display languages than their Wi-Fi-only counterparts, just like on their smartphones.
Certain language packs like English (Australia) and English (Canada) are only supported on the Xbox consoles and the Surface Duo.
Some LIP packs require certain MUI packs (base languages) to be present or compatible. If that base language is not present or compatible, then that LIP cannot be installed on that device.

Beginning with Windows 10 version 1803, Microsoft started referring to language packs as "Local Experience Packs" (LXPs), but they still work in the same way. In addition to downloading from Windows Settings, these 110 LXPs are also available for download through the Microsoft Store app and through the web interface, the latter enabling remote installation for consumer editions of Windows. However, as with any other application from the Microsoft Store, only the LXPs that are compatible with that Windows device are shown through the Microsoft Store app. These LXPs receive updates through the Microsoft Store, outside of the normal Windows update cycle.

List of supported languages 
As of Windows 11, the following language packs are supported.

PCs

Mobile 
The multilingual user interface for Windows Phones did not appear until version 7.0.

See also 
GNU gettext
Language Interface Pack (LIP)

References

External links 
Windows MUI Knowledge Center (archived)
Windows Language Packs

Windows administration