Once content is labelled as being in a different language, it is obvious what texts are in French or in another language. This will not only trigger WebFonts for the languages with a default font, it will also help people analysing the content of the French Wikisource.
WebFonts is a tool that delivers fonts where needed. So far we have not really considered supporting languages in the Latin script with WebFonts. When there are fonts that serve a purpose and are freely licensed, adding support will certainly be considered.
One of the issues that we will have to consider is how to support the really big fonts. When they support multiple scripts it does affect the time it takes to send a font to the user. This is one area where local fonts and web fonts substantially differ.
Historic scripts like Runic and Cuneiform but also modern scripts like the Latin script have evolved a lot over time. To do justice to this and to differences in different places, it will be important to have fonts that reflect these differences.
With the interest of the Wikisourcerers we may learn from them about the fonts available under a free license that do justice to all such considerations.