French Birth Certificate that needs legally translating

There are two issues here:

The first is that, assuming you are referring to a Belgian "birth certificate" in French (rather than one issued in France), it isn't actually a birth certificate in the British sense but rather an extract of the official record of the birth held on Belgium's population register. The important issue being that this extract has a very limited life as a legally valid document, normally only 3 or possibly 6 months. Therefore it is only worth getting a legal translation IF you have an immediate need for it.

The second is that it is normally best to get a LEGALLY VALID translation done in the country where that translation is required as appropriately qualified translators there will know how to get the translation validated. It is easy to get a translation done which is legally validated in Belgium but that validation may well either not be accepted in other countries or need a further level of validation, by the other country's embassy in Belgium, for example.

If you still need a translation done in Belgium, the Yellow Pages are a good starting point for finding a translator.

First and foremost before spending any money, find out exactly what will be acceptable to whomever it is who needs the translation of the original document.
If you want a translation validated by a Belgian court you must use a special translator who is authorised to do this and will charge more than someone who is just doing the translation.

The British Consulate here has an "official list".*

https://www.gov.uk/government/uploads/system/uploads/attachment_data/fil...

* Please note that inclusion in the above lists does not constitute an official endorsement by the British Embassy or the UK government.

If you need the document for an EU country, you don't need a translation. You should go to the commune of the birth and ask for an international birth certificate ( it can be issued in English, French or German). The administrations of all member states are obliged to accept it.