I was having some similar questions (being french as well) regarding client billing.
The Webflow team will tend to answer client billing questions in private probably because there is not a general answer for every country and currency.
You can contact the support team directly by email. By experience their answer is fast and helpful.
Just little bit of information about it is that other currencies than $US is not yet fully supported and may come with the “soon” to be released e-commerce update.
Even if you can display the bill in euros for your client it will be calculated with the current exchange rate.
About the french law I guess you are talking about eventually charging on top of hosting and therefore generate an income with client billing. For now I will look in the same way you would declare incomes from a youtube channel for example with a (pardon my french) “numéro de TVA intracommunautaire”.
I hope those little informations can help you but still I would recommend that you contact Webflow support for this matter or your accounting firm regarding incomes from a foreign country.