Good catch, @Schnark.

Actually a different bug from T60663: media viewer doesn't preserve thumbnail options like svg language (which seems to have been fixed somewhere along the line, at least for SVGs) - the URL guessing provider (mmv.provider.GuessedThumbnailInfo.js) preserves the language but the API-based provider does not. So language detection would work in Wikimedia production but not on most other wikis.

Does the imageinfo API support specifying the SVG language?

For instance, how to adapt https://commons.wikimedia.org/w/api.php?action=query&format=json&formatversion=2&prop=imageinfo&titles=File%3AMultilingual_SVG_example.svg&iiprop=url&iiurlwidth=1920&smaxage=86400&maxage=86400 in order to get the German thumbnail?

@simon04 you essentially have to do a two pass. You need to get the known translations from metadata from it's translations sub attribute

https://commons.wikimedia.org/wiki/Special:ApiSandbox#action=query&format=json&smaxage=86400&maxage=86400&prop=imageinfo&titles=File%3AMultilingual_SVG_example.svg&formatversion=2&iiprop=url%7Cmetadata&iiurlwidth=1920

And then hand specify the thumb parameter prefix, including the width you are targeting and match it to your iiurlwidth

https://commons.wikimedia.org/wiki/Special:ApiSandbox#action=query&format=json&smaxage=86400&maxage=86400&prop=imageinfo&titles=File%3AMultilingual_SVG_example.svg&formatversion=2&iiprop=url&iiurlwidth=1920&iiurlparam=langde-1920px

Change #1036570 had a related patch set uploaded (by Simon04; author: Simon04):

[mediawiki/extensions/MultimediaViewer@master] ThumbnailInfo: support multi lingual SVG

https://gerrit.wikimedia.org/r/1036570

Change #1036570 merged by jenkins-bot:

[mediawiki/extensions/MultimediaViewer@master] ThumbnailInfo: support multi lingual SVG

https://gerrit.wikimedia.org/r/1036570