At the moment I use Sylpheed for emailing and newsgroups (As you might now, Sylpheed is based on Sylpheed-Claws, now called Claws Mail. All following steps are also good for claws).
Sylpheed has a preinstalled en-US dictionary, so every time when I write German mails all words were underlined in red, which is annoying. Claws and Sylpheed use Aspell as spell checker. Additional dictionaries can be found on the website http://aspell.net/
After the download, extract the tar.bz2 file to a folder of your choice. I my case, it was the download folder of the browser. Now open a terminal window, navigate to your folder and type in the following commands:
Which should output something like:
Finding Dictionary file location ... /usr/local/lib/aspell Finding Data file location ... /usr/local/share/aspell
After that build the package with:
and then install it with
Afterwards the new dictionary should be available.