When running QUnit / Selenium tests, the mw-debug-www.log spurts a few traces such as:
LocalisationCache::isExpired(en): cache missing, need to make one LocalisationCache::recache: got localisation for en from source [DBPerformance] Expectation (writes <= 0) by MediaWiki::main not met (actual: 1): query-m: DELETE FROM `l10n_cache` WHERE lc_lang = 'X' [TRX#1254e1] [DBPerformance] Expectation (writes <= 0) by MediaWiki::main not met (actual: 2): query-m: INSERT INTO `l10n_cache` (lc_lang,lc_key,lc_value) VALUES ('X') [TRX#1254e1] [DBPerformance] Expectation (writes <= 0) by MediaWiki::main not met (actual: 3): query-m: INSERT INTO `l10n_cache` (lc_lang,lc_key,lc_value) VALUES ('X') [TRX#1254e1] [DBPerformance] Expectation (writes <= 0) by MediaWiki::main not met (actual: 4): query-m: INSERT INTO `l10n_cache` (lc_lang,lc_key,lc_value) VALUES ('X') [TRX#1254e1] ... [MessageCache] MessageCache::load: Loading en... local cache is empty, global cache is expired/volatile, loading from database
I suspect there is a race condition due to mutliple http queries each triggering LocalisationCache->recache('en'). Potentially after the installation is complete, we might want to invoke php maintenance/rebuildLocalisationCache.php --lang=en.