(bug 1495) Include the site language in the fallback list

In Iaaf6ccebd8c40c9602748c58c3a5c73c29e7aa4d, the behavior of MessageCache
was changed so that it iterates over all possible on-wiki fallbacks including
the site language fallbacks. However, it omitted the site language itself.
Obviously, the problem appears only when the site language is not English,
because English is included always as the fallback language of last resort.
But in other cases, we need to add it explicitly.

Bug: 1495
Change-Id: Ib607a446d3499a3c042dce408db5cbaf12fa9e3d