So Anki makes this annoyingly difficult, and as you've probably noticed, reinstalling Anki doesn't fix it.
To fix it, get to your user folder.
Then go to your Library folder inside there, then go to the Application Support folder inside there.
Then get to the Anki2 folder and find a file called Prefs.db:
Make sure Anki isn't open (if so, quit it). Then delete prefs.db. Open Anki up and you should be able to choose your interface language again.