At present it is possible to set the language in the Composer window and use the Sticky tickbox, but the setting is applied only to the next Composer window. 

In a multi-language situation, it would be more logical to be able to set the language depending on the recipient. If I write to a particular email address, let's say of a German-speaking person, it would be practical to have a setting in the composer to remember that I always want to communicate in German with a German-speaking contact. It would be practical to be able to set this in the composer (and perhaps also store this in the KAddressbook information on contacts that have an entry).  

Reproducible: Always