Add in the address book a field for 'language'. When you select somebody from the address book to send an email the correct language can be used for spell checking.
That would be a nice feature.
I do not maintain an adress book, but of course thunderbird knows the adresses I use. It could also remember the language setting (The last one active, when I sent a message to that guy) for the receipients. Alas, most emails have more than one receipient so that could result in arbitrary language selection. The default from the address book would overrule the automatically determined language setting.
I agree with Aceman, this would be a nice feature. Having had some experience with using the "Automatic Dictionary" addon (https://addons.mozilla.org/en-us/thunderbird/addon/automatic-dictionary-switching) which tries to be very smart, I would implement this with very simple logic: If the first "To" recipient has a language defined in the address book, use this language, if not, use the default spelling language. I wouldn't do any fancy stuff trying to work out the best language for multiple recipients incl. "CC"ed ones. "First recipient sets language" is a simple rule any user will understand. It's easy enough to set the spell checking language, but it would be nice not having to do it for frequent correspondents who differ from the default language. (Note that as of bug 967494 the preference is fixed and doesn't change every time the dictionary is changed.) Note that vCard 4.0 has provision for a "LANG" property (https://en.wikipedia.org/wiki/VCard#Properties), also see: http://tools.ietf.org/html/rfc6350#section-6.4.4. Oliver: Try "Automatic Dictionary".
Since I didn't want to wait for the bug to be fixed, I created an add-on that will set the language based on the first recipient: https://addons.mozilla.org/en-us/thunderbird/addon/dictionary-for-recipient/ The language code needs to be stored in the address book in the "Custom 4" field.
This one might be related to or DUP of "Bug 334320 - Message composition language features should be recipient- or newsgroups-related"