I use alternatively the French and English to fill forms on the web. So I added both languages to epiphany's preferences. What epiphany does is that it checks, for every word, if it belongs to the French *or* English dictionary. There is no possibility to select the language of the form or of the paragraph. (firefox allows this possibility by right-clicking in the form and selecting the language) So if I insert by mistake an English word in a French form, epiphany won't detect it. This has been tested on debian 8 (epiphany 3.14.1, webkit 2.6.2).
I forgot to add the '[gtk]' keyword in the title and now I cannot change it. I hope somebody with the access can do it.