3f5d41d4135740573f642 (bug 32987) introduced support for lang= on localised SVGs.
Thumbnails are generated in this form:
[[File:Gerrit_patchset_25838_test.svg|thumb|lang=de]]
mw.Title.newFromImg(
);File:Langde-345px-Gerrit patchset 25838 test.svg.png
Since both mw.Title.newFromImg and localised SVG thumbnails are introduced recently, this is a 1.22-alpha cycle regression that must be addressed before release.
Version: 1.22.0
Severity: critical