Spain is currently not considered a high-risk country, there is no test or vaccination requirement to enter Belgium, making the question moot as far as Belgium is concerned. If the country were “red”, it would still be possible to enter without a test certificate but you would then be required to quarantine until you get tested.
For France, it's a little murkier. At the moment, coming from either Spain or Belgium, a test is required for people who haven't received a vaccine. Some webpages do hint that what's expected for EU residents and intra-Schengen travel is however the EU digital certificate but it doesn't seem to be explicitly required.
However, the rules on entry are much older and as far as I know separate from the rules on public spaces in France (the pass sanitaire you have heard about). The exact rules fluctuated a bit but some form of test has been required for over a year, long before vaccines or standardised digital certificates became avalaible. In fact, for some countries of origin, the rules are more stringent than those that apply to restaurants and public venues and are implemented in the decoding app (e.g. a test that's less than 24 hours old instead of 72 hours). All this would suggest that what's required is still a proof of test in plain language, not a specific certificate. That's presumably still how many people from outside the EU enter the country.
In both countries, the transportation company is supposed to enforce those rules so you might want to approach the airline to know how they interpret the requirements. Based on a recent experience in Spain, some airlines insist on inspecting the full A4 document rather than scanning the QR code (that was for a vaccinated passenger so a different situation but they did not rely on the digital part of the certificate).
In any case, in France, you are not allowed to take a long-distance train or bus (and therefore to enter by train) without a digital certificate. That's true even if entering by air or private car is allowed in your situation.