Google Voice typing no longer works in French, works in English
I noticed that Android had locally an offline recognition pack for English.  So, I ended up selecting the French language for offline recognition, Android downloaded a pack and now it works.  

Not the actual fix to the initial problem.  It worked online before, but in practice it works.

It does not explain why on-line speech recognition would fail.

Also, is offline recognition as good as online ?

Edit: in order to reproduce the problem with logcat to get a log, I uninstalled offline French pack and now voice typing works at all times, even after rebooting.  So, the problem looks like it's definitely gone and we'll never know?