I followed the installation and configuration instructions on the extension's MediaWiki page, but encountered the following error when browsing to my site's Special:ManageMessageGroups,
[eb0c637e] /index.php/Special:ManageMessageGroups Exception from line 320 of /X/mediawiki-core/extensions/Translate/TranslateUtils.php: $wgCacheDirectory must be configured
Backtrace:
#0 /X/mediawiki-core/extensions/Translate/specials/SpecialManageGroups.php(42): TranslateUtils::cacheFile(string)
#1 /X/mediawiki-core/includes/specialpage/SpecialPage.php(374): SpecialManageGroups->execute(NULL)
#2 /X/mediawiki-core/includes/SpecialPageFactory.php(487): SpecialPage->run(NULL)
#3 /X/mediawiki-core/includes/Wiki.php(298): SpecialPageFactory::executePath(Title, RequestContext)
#4 /X/mediawiki-core/includes/Wiki.php(599): MediaWiki->performRequest()
#5 /X/mediawiki-core/includes/Wiki.php(460): MediaWiki->main()
#6 /X/mediawiki-core/index.php(49): MediaWiki->run()
#7 {main}