If you write in both Esperanto and French,
then I'll understand you better than if
you write in French alone. More generally,
the more Latin-script languages you write,
the better off I am -- unless you write English
(and even then, only if it's good English),
in which case additional languages add nothing.
Supposing that I am typical except (perhaps)
for my set of fluent languages ({English}),
then I'd avise people to write in as many languages
as they find convenient (barring maybe obscure ones
that are unlikely to find readers on the list).
-- Toby