for some reason, your csv file seems to be not valid.

The error output relates to this line: https://github.com/magento/magento2/blob/develop/lib/internal/Magento/Framework/App/Language/Dictionary.php#L183

Each csv row must have at least 2 columns:

"Original String","Translated String"

(Ok I bet you know that)

So you should check the csv File. You could also debug at the given Codeline and try to find out, on which line (in the csv file) this error occurs

for some reason, your csv file seems to be not valid.

The error output relates to this line: https://github.com/magento/magento2/blob/develop/lib/internal/Magento/Framework/App/Language/Dictionary.php#L183

Each csv row must have at least 2 columns:

"Original String","Translated String"

(Ok I bet you know that)

So you should check the csv File. You could also debug at the given Codeline and try to find out, on which line (in the csv file) this error occurs

===UPDATE===

It may also be an encoding Problem. Also, Microsoft Excel for Example almost always broke my csv Files somehow, often just by opening them. I would recommend to use Openoffice or Libreoffice Calc for editing csv files