From Wikitech

Our SVG engine relies on rsvg.

Something critical for SVG support is fonts, which is handled at the OS level. The fonts setup that ensures we have good multilingual SVG support is thus defined in Deployment Charts.

The SVG engine can received information about the "lang" special thumbnail parameter, which allows the client to specify the language to use in the thumbnail, when an SVG is multilingual. This special parameter is passed via our lang custom Thumbor filter.

Our SVG detection is very lax, because the wide variety of SVGs we host has shown that syntax can vary a lot between SVGs and that valid XML isn't a requirement to render an SVG.