This page explains how to add a dictionary to the Evolution spell checking collection.
Please follow these steps carefully :
First go to the synaptic package manager (Men System>Administration) and search for Aspell in all the available packages. Check the list and install all those that you want to integrate.
Then restart Evolution if you where using it and go to the Edit -> Preferences -> Composer Preferences -> Spell Checking
You should see all the languages you have installed. Check only the languages you use regularly in your messages as the unchecked languages can be activated from time to time directly from the message your typing in (Edit>Current Languages).
Then open a new message and verify that the spell check works correctly.