OK, I understand that
db.version() is a Javascript function, and so, is not available as a MongoDB command.
Although these methods use JavaScript, most interactions with MongoDB do not use JavaScript but use an idiomatic driver in the language of the interacting application.
I had the hope that reactiveMongo driver has implemented an equivalent method.
Anyway, I've finally used
buildInfo MongoDB command which returns version among other useless (in my case) informations.
Thanks for your help.
Guillaume