Is it ok to insulate external cantilever floor joists with foam board to fix cold floor

Yes, Rigid foam (XPS and ISO) boards are frequently used in exterior applications, and can be a great option for improving efficiency. There are a couple of issues with your proposed approach, though:

1. Vapor barrier location. The vapor barrier should be on the warm side (probably inside, unless you live in a very hot & humid climate). Ideally, this is immediately behind your interior drywall. You also do not usually need an extra vapor barrier layer adjacent to rigid foam sheets, they are typically highly resistant to moisture.
2. Attachment. You cannot rely on glue to support foam and siding panels. These need to be mechanically fastened.

On a wood frame house, you'd want to use this order:

• interior drywall
• possibly a vapor barrier (e.g. poly sheeting), though the wisdom of this is debatable (see the comment from iLikeDirt below)
• wall studs (optionally with batt or cellulose insulation in between)
• exterior sheathing
• rigid foam
• wooden strapping for attaching siding, screwed to studs through foam. This provides support for the siding and also allows air circulation behind the siding, so any moisture that gets behind the siding can dry out.
• siding panels, nailed to strapping

Here's a visual from Fine Homebuilding, which has a couple of good articles on this:

Note that adding foam on the exterior of your house complicates some things. You will likely need to redo the trim on your windows and doors, which will become more in-set. These inset areas will need to be flashed properly or you will have leaks, which have more potential to do damage when you've got multi-layered, thick walls that could trap moisture. If done well, you can get very good insulation. But make sure to work with someone who knows what they're doing!

Buildingscience.com has a decent article that covers a very similar situation with floors over crawlspaces

The two key points with this:

• One vapour barrier, so as not to trap moisture. Essentially, drying happens on both sides going in the direction away from the vapour barrier, and if you use two barriers you're trapping moisture which leads to mold and rot.
• Condensation happens when warm meets cold. When you're using batt insulation, you want the vapour barrier on the warm side so you're not getting condensation (and thus water) on the insulation.

The situation that most closely matches what you're talking about recommends this:

The rigid foam on the bottom of the joists is your vapor barrier: it must be impermeable and this means that the connections between sheets and around the edges must be sealed too. Even though the vapour barrier is essentially on the outside of the batt insulation, because the foam has an insulating value (unlike 6mil plastic) you don't have warm meeting cold here and won't get the condensation problem.