From Winamp Developer Wiki
Interface name: api_language
Include file: Agave/Language/api_language.h
Related files: Agave/Language/lang.h
GUID name: languageApiGUID
api_language allows you to localise Winamp via the WLZ language packs (introduced with v5.5+) which provides the means for a more complete translation of the player and its skins unlike the 'hacks' used previously and the limited single language file for just the winamp.exe resources.
Uses by Winamp:
It is used throughout the player from loading of preference dialogs to the display of anything text based as long as the skin or plugin is designed to work with it.
Although the api is based on a Wasabi service, there is also IPC_GETLANGUAGEPACKINSTANCE (expanded with 5.5+ - listed in wa_ipc.h) which allows you to query certain details about the currently used language pack (if one is active) making it easier for 3rd party plugins to access localised resources from the language pack if they do not or cannot make use of the api_language interfaces directly.
How to use: