Extract list of languages svg is translated into.

SVG files support having different renderings based on language.
We support changing the rendering language, but we offer no
discoverability of what languages are available.

Long term Jarry's TranslateSVG will hopefully be used and have
all the associated awesomeness. In the mean time, we should
probably have some indication on the image page what languages
are available for the SVG. This is step 1 (extract the language).
I expect to follow this up with a commit adding some sort of
simple selector on the image page.

As an aside, it should be noted, this only detects languages
if its in the first $wgSVGMetadataCutoff (256 kb). This is a
compromise to avoid OOM on huge SVG files.

Change-Id: I2a462270fe63eb3e3023419eddc8b06f5a617ab5