Don't use $wgVariantArticlePath for variants of non-content languages

Currently PathRouter understands variants of $wgContLang only, and it
wouldn't be easy to extend it to variants of the given title because at
the time of preparing PathRouter for parsing, the title is unknown yet.

Bug: 52852
Bug: 52850
Change-Id: I7dcf7dc73e36945e420a66d99329c09c75b17882
Merge "Don't use $wgVariantArticlePath for variants of non-content languages"