thunderbird: set default dictionary

Setting the default language for spelling checking in the new message editor in Thunderbird:

Menu - Settings - Composition - Spelling - Language

I had the same problem for some time. For me what worked was installing again the diccionary that I want as default and then selecting them in the Options:

Menu -> Tools -> Options -> Composition -> Spelling -> Language

It is also from this very same place whence you can download and install the dictionary you need.