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If you can't get into the admin area, then I suggest either (i) restoring your last backup or (ii) using PHPMyAdmin to manipulate the database directly and change the language back. If you have a look through the preferences tables you should be able to find the language setting.
Edit: Or iii) use PHPMyAdmin to turn debug mode on, then you'll be able to see the errors that are generated and perhaps that will give you some other lead.