MediaWiki serves SVGs directly to browsers when $wgSVGNativeRendering is set to true. For multilingual SVGs that use switches with systemLanguage attributes, this leaves language selection to the user agent. Ideally, the rendered language should be the page content language or the lang= attribute of the thumbnail, not the browser/client language. Setting $wgSVGNativeRendering to 'partial' avoids the problem but only by rendering PNGs.

Proposed solution: if the file is less than $wgSVGNativeRenderingSizeLimit, it should be safe to read and manipulate it in-memory. We can generate a new SVG in which all nodes with non-matching systemLanguage attribute are removed, and store it in the file repo similar to how PNG thumbnails are stored.