This was added to the PHP parser in bug 32987. Basic parsoid support was added in change Iae50f6e4948844b94a66f8437e12e05aa3ec1685.
Currently Parsoid set the lang attribute on the <img> and references the SVG directly in the <img> tag's src attribute. This is semantically correct.
However, in my tests, Firefox and Chrome (and all other browsers?) ignore the current lang when directly embedding SVG content.
Further, the SVG spec itself appears to be somewhat broken: http://www.w3.org/TR/SVG11/struct.html#SystemLanguageAttribute says that the system language should be *operating system's* language, not the current document language. Therefore, if I'm viewing he.wikipedia.org from my standard en desktop, all the SVGs will still be in English. That seems wrong.
So there are two wrongs here -- Parsoid isn't implementing this exactly the same as the PHP parser, and the PHP parser appears to be misusing the systemLanguage attribute. Hopefully someone will straighten this out eventually.
T18052: Support for multilingual SVGs