Aspell is a handy-dandy tool for the command line for spell checking text documents. When you're editing Tex documents, Aspell is something you really want to have. However, as a German, installing Aspell on a Mac has one big pitfall: it won't install German per default.
What I (foolishly) did up to now is
brew install aspell --all
which installs all languages Aspell knows. This is quite a lot. So installation takes days. OK, a couple of minutes. But chances are rare that I need to spell check a document in Farsi or even more exotic languages such as French.
So better execute
brew install aspell --with-lang-de --with-lang-en
In order to receive the list of all installable languages execute
brew options aspell
Now you can check documents in German with
aspell --lang="de_DE" -c FILENAME
A list of your installed languages can be retrieved with
aspell dump dicts
Unfortunately, it seems to be impossible to add a dictionary after having installed Aspell plus your first dictionaries. To add the dictionary you have to uninstall Aspell and install it again together with the needed language.