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`.