Offline Spanish voice recognition: "no connection for speech input (E4)"
Aha! Five minutes later... It's the setting in the Google Translate app. Settings - speech input - Spanish was set to Default Dialect. I set it to Spanish (Spain) and voice input now works on both devices in Spanish in Flight mode.

It would be be nice if Android was a bit less picky when it came to missing language data, like web pages work with fallback fonts. But, it works, quite well, and it's free!