Level of French needed for Belgian passport

None whatsoever. If you have been working here and can show that you have been paying social security, either as a freelancer or as an employee (I think it's around 500 days over the past 5 years), then you meet the criteria for social and economic integration and you therefore don't have to do a language test.

You also don't need to meet the language qualifying standards if you have reached the Belgian retirement age (currently 65) or if you are handicapped.

If you do need to meet it, the level is A2, which, frankly isn't very high, and the commune will explain where you can get a test certificate.

Just a note - the language required depends upon the commune where you have been resident more than 5 years (which should be the commune where you lodge your application). If you apply in a Flemish commune then you need proficiency in Dutch (not French).

Both the 5 and 10 year qualification options require knowledge of 1 of the 3 national languages at level A2. Not sure if J means you don't need to do a test or don't need the language in the first place.

Maybe Rachxpat has personal experience, but the commune acts as a gatekeeper to check paperwork and eligibility. The nationality process and decision lies with the federal body, which will recognise all 3 languages. Of course your commune may insist on a communication language and some more protectionist communes might try to tell you otherwise, but they can't impose this language upon the language knowledge element of your application.

Sorry - meant to add: Both the 5 and 10 year qualification options require knowledge of 1 of the 3 national languages at level A2 and proof of this knowledge (in the official process). This needs to be a document stating your level issued by a Belgian institution. My commune will not accept documents from private schools such as Berlitz - they require a school or test recognised by the commune - e.g. CVO, Huis van het Nederlands etc.

As stated by "J", no language certificate is required if you qualify under the continuous employment rule:
http://www.a-law.eu/nl/node/123

Where certificated knowledge of the language is required, it MUST be the official language(s) of the commune concerned, that is the national requirement and not a rule made up by an individual commune.

The point with the over-riding "employed" exemption is that you will already have contributed handsomely to Belgian coffers and, on becoming a national, can be expected to continue to do so in the future.

The A2 language level required really is quite low, if you can communicate in shops or if you visit the commune, you are probably already there, and quite possibly well on the way to B1. It is the shift beyond B2 that is difficult unless you are using the language continually or are "gifted".

Again - the requirement is to provide certified knowledge of 1 of the 3 NATIONAL languages, independent of the commune in which you reside. Feel free to prove me wrong if you can find an official reference but official government websites as well as my own commune state this.

Additionally, there may have been a court ruling in Gent on the 5 years language question, but you'll need to persuade your own commune to forgo their usual rules and accept this - suggest printing out anything you can find on this ruling and taking it with you. Ask the commune - ultimately you can argue all you like with the commune on the basis of our various answers, but if the commune have an inflexible line on what they require from you, then you're just making the process harder on yourself.

Indeed because you are wrong all docs and language requirements are any of 3 national languages. I submitted in French and Dutch.