Use $wgTranslatePageTranslationULS

Use case: see Quim's https://bugzilla.wikimedia.org/show_bug.cgi?id=61695#c5

This will help in the case when all the following is true:
* the user enters a page with a direct link without (correct) language specified;
* the page is translatable and ULS is prominently available ("personal");
* the automatic language detection fails (e.g. the user doesn't have a properly
  configured accept-language so MediaWiki can't redirect to the proper version
  of the main page as defined in MediaWiki:Mainpage translations;
* the user is logged in (pending bug 56464);
* the user manually selects the preferred interface language via ULS;
* all the following links use Special:MyLanguage (see [[mw:MyLanguage]]) to keep
  the content language constant, i.e. equal to interface language.

It's a edge case but worth addressing with this minor configuration improvemment.
The true difference, of course, would be bug 56464; until then, ugly selectors of
all sorts will still be needed everywhere, including mediawiki.org's main page.

Change-Id: If405735881113198cd713d81d8c5dafc782df264