Anki makes this annoyingly difficult, and as you've probably noticed, reinstalling Anki doesn't fix it.
To fix it:
- Go to your User Folder.
- Go to your Library Folder inside your User Folder
- Go to the Application Support Folder inside your Library Folder.
- Go to the Anki2 Folder and find a file called Prefs.db
- Make sure Anki isn't open (if so, stop it completely).
- Delete prefs.db.
- Open Anki up and you should be able to choose your interface language
This is the only way to change the language in Anki after you have chosen the wrong one.