Hello Wouter,
wikipedia-l explicitly allows posts in languages other than English,
especially since intlwiki-l was merged into it. However, list members
are encouraged to translate messages from one language into the other.
This is a bit messy given the limits of the medium, but it's certainly
possible - just post a translation as a reply.
There are quite a few people for whom writing in English is a lot of
effort, just like writing in French is for you and me. And of course
there are people who don't speak English at all. These people should not
be excluded -- they should be able to start discussions, which could be
opened up to a larger group by anyone reading this list and being able
to translate them into other languages.
Of course, if you speak English, using it is a good way to reach the
largest number of readers possible. But that is a decision that should
be up to the original author of a message.
NB: It appears that you sent something which was sent to you off-list to
the public mailing list. Subjects like OFF or OFFLIST are generally used
to indicate that something is private and not meant to be shared.
Best,
Erik