Translation and localization is important in general, because it allows people who do not have knowledge of English to use computer software. MediaWiki is used for example to run Wikipedia in many languages, and therefore translating MediaWiki directly helps people to access free knowledge in their own language.
Here at translatewiki.net we are trying to build infrastructure for easy localization of MediaWiki. You do not need to know PHP programming language and its syntax, which may be hard to understand. If you are familiar with MediaWiki, you will have no problems with translating.
MediaWiki is currently used in more than 250 languages, and only a few dozen of those languages are maintained. See current status of translations.
Next step: How to start