Manual:$wgInterwikiCache

Details
1>Special:MyLanguage/Interwiki cache|Interwiki caching consists of the following four settings:


 * This setting specifies an array in the following format, or a string with the path to  file containing the same keys. This format can be generated with .
 * dbname:key - a simple key (e.g. enwiki:meta)
 * _sitename:key - site-scope key (e.g. wiktionary:meta)
 * __global:key - global-scope key (e.g. __global:meta)
 * __sites:dbname - site mapping (e.g. __sites:enwiki)
 * Sites mapping just specifies site name, other keys provide "local url" data layout.
 * Sites mapping just specifies site name, other keys provide "local url" data layout.


 * This setting specifies the number of domains to check for messages:
 * - Just wiki(db)-level
 * - wiki and global levels
 * - site levels
 * - site levels


 * This setting is used if unable to resolve from the cache.
 * This setting is used if unable to resolve from the cache.

Since  it is possible to get the list of cached interwiki prefixes, used in the 2>Special:MyLanguage/API:Siteinfo|API and  extension.