I decided to appeal the fine as Under European Union regulations, particularly the EU Cross-Border Enforcement Directive, authorities are generally required to send notifications of traffic fines in a language that the recipient can reasonably understand, which should ideally be the language of the country where the vehicle is registered (in this case, Portuguese).
I don't think there's a legally enforceable reason for them to assume that i can understand the contents of a letter sent to me in French, which i can't just that it's some sort of fine, and that would make the enforcement of the letter dubious.
I'm going to send an appeal telling them that i can't understand their letter and suggesting they communicate in Portuguese, or through the Portuguese authorities. Also letting them know that i'm mostly living in a camper van and rarely at the address they are communicating with and so am unlikely to be able to meet the deadlines they appear to be imposing, to try and get some leeway regarding the extra fines.