I finally solved the problem. It's related to [a
bug](https://bugzilla.mozilla.org/show_bug.cgi?id=1140831) in the i18n
stuff. I fixed it by lanching the english version, fix the bug and
restart thunderbird.
And even the existence of the package is dabatable: it is just a perl wrapper
script hosted in AUR to let the user _interactively_ (bad!) select the language
in which she wants to use thunderbird. It then downloads a binary version of
thunderbird and creates a package.
I agree, but maintaining a package per language seems not a good
solution to me.
Current pkgver is 36.something, not 37 -- maybe for a reason? Ask the
maintainer.
I updated the version manually, as the package has been out of date for
a few days.
