Usually this error means that you could have been changing between different languages (locales) and something has caused this to error erroneously.

You could try regenerating your list of locales with

sudo dpkg-reconfigure locales

For me the result was:

Generating locales...

  en_AG.UTF-8... done

  en_AU.UTF-8... done

  en_BW.UTF-8... done

  en_CA.UTF-8... done

  en_DK.UTF-8... done

  en_GB.UTF-8... done

  en_HK.UTF-8... done

  en_IE.UTF-8... done

  en_IN.UTF-8... done

  en_NG.UTF-8... done

  en_NZ.UTF-8... done

  en_PH.UTF-8... done

  en_SG.UTF-8... done

  en_US.UTF-8... up-to-date

  en_ZA.UTF-8... done

  en_ZM.UTF-8... done

  en_ZW.UTF-8... done

Generation complete.

Then resetting your local locale with:

sudo update-locale LANG=en_US.UTF-8

i.e. use one of the Locale values in the output above