After messing around in the spellchecker's preferences, I found that Apple includes a "Multilingual" dictionary.
Simply open any app that supports spell checking, select "Spelling" from the edit menu, and "Spelling" from the submenu, and in the pane that appears, set the dictionary to "Multilingual." Words in all languages should now be checked correctly.
Unfortunately, there is not a dictionary for every language Apple supported, but there are quite a few of them.

