Don't localize the namespace unnecessarily

When the source title is set, mw.Title localizes
the namespace name, which is incorrect, because
the namespace names are usually different in source and target languages.
This is especially felt when translating non-main-space pages
from the English Wikipedia, where the namespace names are canonical,
and are unnecessarily translated.

Bug: T86744
Change-Id: I6c3996986d7b6f5bf0757c17e6dd505a0c6b0feb