i18n: consistency tweaks

* replace single-quote by double
* replace "millennia" by "millennium"

Spotted by Siebrand
https://translatewiki.net/wiki/Thread:Support/About_MediaWiki:Wikibase-item-summary-wbmergeitems-from/en
https://translatewiki.net/wiki/Thread:Support/About_MediaWiki:Wikibase-item-summary-wbmergeitems-from/en_(2)
https://translatewiki.net/wiki/Thread:Support/About_MediaWiki:Wikibase-item-summary-wbmergeitems-to/en_(5)
https://translatewiki.net/wiki/Thread:Support/About_MediaWiki:Wikibase-listdatatypes-time-body/en

Change-Id: I09ca834dcc096125df573161c6edbc9f2543b516