Partly disable title conversion if variant == main language code

Partly disable title conversion if variant == main language code

Currently the main language code means "no conversion",
but may technically be used as a standalone variant (and
usually falls back to another variant). However for the
page title, since there can be a nice unconverted version
from real page name, it seems better to provide people who
choose "no conversion" with this unconverted version, rather
than a fallback from a variant which is always converted.

If a string targeting exactly the main language code exists
(ie. no fallbacks needed), it's still used. Because the
magic word DISPLAYTITLE cannot be used together with
language conversion syntax and people must use -{T| }- to
show a modified display title for "no conversion" version.

Change-Id: I25cf5a94f6e47f85a9d0b80cc1c9c9f957288478