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