This plugin will display a small '?' icon that links to either the definition or the translation of the word. The plugin is a no-brainer to use:
[define:dog] will define the word 'dog' and
[en2fr:dog] will translate the word 'dog' from english to french. The plugin is language-sensitive; Spanish installation will return spanish definitions. Google Dictionary is used to define words, as it has definitions for technical terms in multiple languages. There is also built-in support for translation of the plugin.