Graphical challenges:

A challenge can be in English and still have problems but what does an English speaking user do if he or she gets an email with a URL pointing to a challenge that looks like this? ... or one with a different URL pointing to something like this?

... and what does a Chinese or Russian or any other person who doesn't speak or read English do if he/she gets the first challenge above?

Text challenges:

Text-based challenges avoid the visual problems faced by the blind, the colour-blind, those with text-only browsers and those with monochrome monitors but a challenge in English still presents a language barrier to those who don't speak English. And a challenge in some other language,
  German,
    Spanish,
      Japanese or
        Chinese
would still present a barrier to those who speak only English.

There are, in fact, other problems with challenges. Besides erecting a barrier to communications when challenges are sent to the correct address but in the wrong language, when sent to a forged address how is the recipient of the challenge going to distinguish a challenge from a spam when he or she doesn't speak the language? And how does the challenger avoid sending challenges to forged addresses?