In general, MediaWiki tries to follow Unicode. We could debate forever on what to do with devices which have poor support for i18n. This is a matter of style, not semantics, so we ought to be consistent (ideally) but at the same there is no urgency justifying a mass edit war.
I use DjVu Serif and an edit like special:diff/9131394 adds a space for me before the colon, where previously there was none. It's clear that the translations did not follow a consistent rule.
I urge the French translators to have a discussion on portal talk:fr (in French is better), involving all those who used or did not use the various kinds of spaces in the past. It's important to find a consensus on 1) what space would be preferrable in an ideal world, 2) what space is recommended right now (this should be written on portal:fr so future translators can learn about it), 3) whether it's ok to perform mass changes to impose the recommended style.
I suspect the solution will come from software, just like we have a special exception in the MediaWiki parser to deal with French spaces. After you have a consensus on what an ideal world would look like, please post a feature request in MediaWiki-Internationalisation, so the devs can see whether spaces can be added/standardised on the display side (some even use CSS for this purpose, I believe), possibly based on the device as well.