That would only appear if you put a file named square_handler.php into the /includes/modules/payment directory. The system looks for the module files, and their corresponding matching filename for language file. That error message means you've got a module file present without a matching language file.
And in this situation you should not have a "square_handler.php" file in the /includes/modules/payment directory. It belongs in the root directory of your shop.
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