I do often use Kate to write stuff in either French or English, and I use the spellchecker. However, switching language is a real pain : one needs to open Tools > Spelling > Change Dictionary, and then only can select a dictionary. It would be much easier (and logical) to have a Dictionary combo next to the Encoding and Syntax Highlighting ones. Thanks for your great software ! :-) Reproducible: Always I'm using Project Neon packages in KVM, but I doubt this info is relevant...
A new feature of Sonnet in the KF5 land is automatic language detection: http://martinsandsmark.wordpress.com/2014/09/23/automated-language-detection/ I'm not sure adding another combo box to the statusbar is a good idea, since the statusbar gets more and more cluttered then. Would the automatic language detection already help?
It would definitely help if it works well, as it's a more elegant solution to the same problem. (I find the combo that appears when one selects "Change Dictionary" hard to discover, by the way.)
Closing since Sonnet does indeed work well.