How to enable spell checking on the Evolution mail client on Kubuntu:
Install the aspell package (normally is installed) and the aspell language files for your idiom. In my case aspell-pt-pt. Then you also need to install the gnome-spell package so Evolution can access the dictionary files.
Go to the evolution client and select Edit->Preferences->Composer Preferences and finally spell checking. The installed dictionaries should be available.
Now ant time you write a message, if a word is underlined by a curly red line, just right click, and at the middle of the menu it should appear the languages to be used to spell check the word.