Make Wikibase diff table to match user language direction

This overrides getDiffLang which is used on parent class addHeader
for choosing right language direction to change diff-contentalign-left
to right for users with RTL UI

By default diff table use language of title which is not very useful
on Wikidata which most of its items and properties is translated

Change-Id: I0567dde13fc0c9cbe31a2cc38f1dc76216aca00f
Merge "Make Wikibase diff table to match user language direction"