It seems there is no detection mechanism in place thats why he can do it without any issue. That has me wondering why not more people do it?

What is your objective here? what outcome would you want to happen?

Why is it difficult to recover the lost working hours? If it's a 1 hour/week course are you able to stay an extra 30 minutes twice per week?

It's quite more then one hour per week, can be 2h...4h per week.

The best outcome would be to obtain the same advantage, but "the proper way"

Well the rules seems perfectly clear and from where I see it, logic. If company is already paying for lessons, it seems logic they won't pay also for your time not working. If this time is so hard for you to catch up (if you ask me 4 extra hours a week is not that hard to do), you shouldn't spend it, so long for the language lessons... If you want to get the butter and money for the butter (as a French you'll understand what I mean), I don't think anyone here can help...

Friend, I do thee no wrong: didst not thou agree with me for a penny? Take what is thine and go thy way

If you were to not put in extra time and consistently do your 40 hours minus the time spent on this course, would that trigger management questions?

Sure it will, for those 40h I've signed a contract.

Laurent S: Totally agree, this was part of "minding my own business"

Ask HR to send a reminder of the rules to all staff?

@LaconicDroid - I really disagree with that approach... but it's an actual answer to the OP's question. Why not write it up and give some reasons why you think it's the right move?

Very related, possible duplicate: What can I do to make a coworker's lack of effort more visible?

@Lucas Sorry so by "If you drop below a certain (generous) level" you mean 40 hours? It would be much clearer if you just said "If you drop below 40 hours per week".

@LaurentS. why do you think that its fair for company training not to be paid or counted (for salaried employees) I doubt that would be legal in France.

IANAL, but... In some European countries (e.g. Germany), what your coworker does is considered to be fraud ("Arbeitszeitbetrug") and a viable reason to to be fired on the spot (even in countries with strong employment protection laws like Germany). It doesn't really matter whether he recieved the email or not, since he does seem to know perfectly well about the rule and merely decided to not obey it.

@Neuromancer > This isn't company training the way I see it: it's not mandatory it's a service offered to the employees willing to attend. If the OP should take these courses out of his own initiative, he'd have to do it after hours at his own expenses. As the company is already paying for the courses and "offers" the premise, I think it's kind of fair they're not the ones to pay "twice": once for the courses and once again for the unworked-but-paid time of employees (5 to 10% of working hours). I don't know for France but this is certainly legal and actually quite common in Belgium.

IMHO just "snitch" on your coworker to HR. Company rules should be company rules. If somebody does not follow them it is legitimate to go to HR and ask why & whether you should still follow the rule too. Obviously this will only end up removing the benefit to your coworker and your envy towards him, it wont magically give you 4 hours worked extra per week (except if HR for some reasons decides to change their policy)

How's your colleague's productivity? Are they getting a week's worth of work done in fewer than 40 hours?

@Lucas Can you please state your goal in this situation? So far your entire post is just a rant.

I agree with @LaconicDroid. Contact HR to clarify the rule they are making you comply with. Ask them if it applies to everyone without exception. Suggest to them that not everyone may aware of the rule(don't mention specific people), and that they may need to send a company wide reminder of the rule. There's not much more you can do beyond that without coming off as a d*ck.