Outplacement

You shouldn't be paying for the outplacement.
Go back to the employer. With a union rep if you have one.

As of 2016, any employee with a theoretical notice period of more than 30 weeks will have 4 weeks deducted from their severance pay regardless of whether or not they actually decide to follow the outplacement offered by the employer. That part is legal, even obligatory.
What do you mean when you say the outplacement company did not want to work with you? This strikes me as irregular. What is the reason for this? Normally your employer has paid for your outplacement.

What a wonderful system. Large chunks of your severance confiscated to pay for something you may not want or need?

Go back and speak to the company and point out you have paid and you expect to be offered a service as is obligated.

@Mike, to be honest, the way that severance pay is calculated in Belgium is quite generous, and the goal of outplacement is to keep people off the dole. And if you're the type that doesn't need it at all because you've already found a new job, 26+ weeks instead of 30+ weeks of severance pay is not going to financially hurt you in the slightest. I don't think it's such a bad system, except of course that it makes payroll even more complicated - which is a good summary of most every aspect of Belgian social legislation.

@JacqueP, normally your employer is legally required to withhold the 4 weeks unless they're feeling generous. However, they are also required to offer you suitable outplacement, so I think it's fair that they give you the 4 weeks and an amended C4 (I don't know by heart but I think it needs to be mentioned in the 'remarks' section at least) unless they offer you an alternative. Basically, in your situation you have the right to outplacement and I would indeed contact your (or a) union to intervene on your behalf.
Finally, I would follow Dutch and/or French courses if fluency is an issue because it's only in specific positions in certain international companies that you will be able to get away with only properly knowing English.

SD.....The fact the sytem is generous is completely and utterly irrelevant. I have been taxed to death for years and will be taxed to death on my severance at the end of the year. To take another 4 weeks pay to provide a service I do not need or want us simply adding insult to injury. It's ok because I can afford it? Oh please!

And it would appear that while the money is gone from your redundancy package, they suggest you look elsewhere if you don't have fluent French or Dutch? Am I reading this right?

It certainly seems relevant to me. In many other countries with the same seniority you would not be entitled to any severance pay at all, let alone 26+ weeks' worth of pay (which is also generously defined, including any type of benefit that you may have). Your employer is not offering it to you out of the goodness of their hearts and in fact they are completely free to give you the full 30+ weeks or however much extra they want. Perhaps something to consider before you complain about the nanny state 'taking' something from you.