The language used to render SVGs should be definable

The language used to render SVGs should be definable

See bug for context.

The implementation is slightly untidy because I've written it so
as to avoid invalidating the existing SVG thumbs -- there will be
no immediate difference (visual/performance/other) as a result of
this.

Tested by me in both...
* [[File:Example.svg|thumb|lang=fr]] AND
* http://example.org/w/index.php?title=File:Example.svg&lang=fr
...modes. Example file on
https://commons.wikimedia.org/wiki/File:Gerrit_patchset_25838_test.svg

Added parser tests.

Bug: 32987
Change-Id: I4cadf96ecd5e169a88ad468a0478d355db980103