Evolution uses gnome-spell, and it uses aspell on my box, so installing appropriate aspell-xx package into your system (I guess aspell-es) and choose in Edit->Preferences->Composer Preferences, tab Spell Checking that language should work for you. You can change it in composer window itself in Edit->Current Languages. I'm not sure how it works on ubuntu, but should very similar (the package name, I meant).
Indeed they are the same. sudo apt-get install gnome-spell aspell aspell-xx -- ..hggdh..
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