Usually this error presents itself in the event you were to edit a language file with a text editor, such as Microsoft's® Notepad.
Some applications insert a signature or Byte Order Mark (BOM) at the beginning of UTF-8 text. For example, Microsoft Notepad will always adds a BOM when saving as UTF-8.
Older text editors or browsers will display the BOM as a blank line on-screen, others will display unexpected characters, such as "ï»¿". This may also occur in the latest browsers if a file that starts with a BOM is included into another file by PHP.
I would recommend Notepad2 to remove the BOM. Notepad2 is a text-editor that enable fast converting between different encoding and line-ending types, and support highlight for many languages. It's very light and a portable application (doesn't need to be installed). Other alternatives that have reported to work are Wordpad, Dreamweaver (paid), Notepad++, and many more.
Another solution is to use a PHP function, it's also more convenient to convert many files with a PHP functions.
Here is a nice explanation about the UTF-8 BOM and also a PHP function that remove the BOM (I haven't test it): Develobert Blog