Ubuntu is having powerful applications in the default installation. Dictionary is one such. You can find Dictionary application in Ubuntu main menu -> Applications -> Office -> Dictionary. This application needs internet connection since it fetches data from dict.org server.
By default this application is English – English dictionary. It is also capable of supporting many other major languages in the World (But still Tamil is missing ). Here is how to select different language dictionary.
Click Ubuntu main menu -> Applications -> Office -> Dictionary -> Edit -> Preference to select a Dictionary Server. Ensure that you have selected default dictionary server to support multiple languages. If you want only Spanish dictionary, please select Spanish Dictionaries in the Source list. I have selected Default Dictionary Server in this post.
Now in main window, click Similar words and change it as Available Dictionaries. It should list different languages dictionary.
Select your language of choice. Enter the word in Look up: text box and press Enter.
Right now it seems to be Hindi is the only Indian language supported.
I am expecting a day when Tamil is supported. Since the time is riping for Cloud, you can check out English – Tamil and Tamil – English dictionary by Google.