I thought from what you said that this was a repeated pattern of behaviour. If this is the first time he's done this I can't see any particular reasons to be dead against it. That said I would expect some specific goals from it, not just a vague idea.
It's supposed to be a investment into his professional future.
How exactly? Is there a job he wants which requires French language skills? Is there a sector he'd like to move into which requires it? Does his company deal with French contractors/clients a lot? Are there better opportunities for his career in France?
I have been retraining recently, and it involved cutting down on my hours at work to study for a qualification. There is a specific sector I want to move into which this qualification will allow me to do. I discussed it with my partner, looked into how much it cost, and we worked out how to pay for it.
Sit down with your budget, work out how you'll afford it, work out how you'll deal with things like childcare while he studies (there will be 'homework' too I expect), what will you do if he doesn't pass? What if he doesn't like the course?
In theory though, as long as there is a specific reason for doing it, you can afford it and have childcare covered I can't see why not.