Disguise original-bucket Language so Jellyfin doesn't drop it.
ProviderManager.GetImages() filters the returned RemoteImageInfo list to {empty, preferred, "en"} when the UI's "All languages" toggle is off (the default), then sorts what survives via OrderByLanguageDescending(preferred) which puts preferred first and English second. Both behaviors hide the original-language bucket, e.g. Japanese posters never reach the user even when IncludeOriginalLanguage is on.
Workaround: relabel original-language entries with the preferred ISO code in the RemoteImageInfo.Language field. The image survives Jellyfin's filter and lands in its "preferred" sort bucket alongside real preferred matches; LINQ's stable sort keeps our internal preferred, original, fallback order.
Tradeoff documented in code: a Japanese poster shows "DE" in the UI label. Functional, white-lied.