Transitional patch for bug 34832: introduce a CI-style option to allow deployment...

Transitional patch for bug 34832: introduce a CI-style option to allow deployment of 1.19 to converter wikis without disruption. The bug fix can be rolled out later by setting $wgBug34832TransitionalRollback = false. This is meant as a temporary measure, while we figure out a way to properly support Chinese wikis for inclusion in the 1.19 tarball.

Introduce a global variable which causes language conversion to not be disabled in interface messages (as before r94279). Use $wgContLang for conversion (as before r97849) since $wgContLang is set to the base language (e.g. zh) on converter wikis, whereas a typical user language (e.g. zh-tw) only has a FakeConverter.