Error messages are shown for various issues with input parameters on thumb.php, and three of them are localized (with the badtitletext message). All others are in English only.

An invalid title has a localized error: https://example.org/w/thumb.php?f=Test[].svg&w=600&uselang=fr

Error generating thumbnail
Le titre de la page demandée est non valide, vide, ou encore mal formé s’il s’agit d’un titre inter-langue ou inter-wiki. Il contient peut-être un ou plusieurs caractères qui ne peuvent pas être utilisés dans les titres.

But a missing width param does not: https://example.org/w/thumb.php?f=Test.svg&uselang=fr

Error generating thumbnail
The specified thumbnail parameters are not valid: Empty return from File::thumbName

It looks like the localised ones are the inconsistency, given that the page title ("Error generating thumbnail") is not.

The occurrences of badtitletext should be replaced by English messages (and also perhaps be different, as they apply to different error states).