MediaWiki is the software used by the projects of the Wikimedia Foundation. If there is one thing MediaWiki can boast about, it is the amount of localising that has been done. There are currently projects in 250 languages and, many have been localised somehow somewhere.
The Wikimedia Foundation has a big need for money. If ever there was an organisation that took care of the money it received it is the WMF. Wikipedia has according to Alexa the 12th traffic rank at the moment and the growth of their projects is something like doubling every four to six months.
The WMF has an ongoing funding drive, they created software to manage this in Drupal. The functionality is good for some languages but for others Drupal has not been localised. Some will say: it does a good job for my language but for most languages Drupal is just not up to the task. MediaWiki is far ahead of the pack when it comes to localisation and consequently there are no tools that will support the languages that are needed by the Wikimedia Foundation.
When Drupal gets more localisation done, it will find that in the same time frame the WMF will have added more languages. There is a moral question here as well. Is it acceptable to use tools that convey an important message that do not support the languages that you need..