Ok. Here are the tips/suggestions from my side.
1. Design your GUI. I mean the user interface to give the input and to display the output.
2. Come up with the list of online dictionaries you are interested in. Find out how to use their service. If it is HTTP call then you may directly invoke it. Otherwise you may need to get their API to invoke their methods.
3. Whatever results you get every time you can cache it into your database.
4. Localization - I think you should skip it as of now.
You will need to use several technologies - JDBC, Swings, Java Socket/Network Programming, etc.
If you want you can also go for frameworks.
First I would say find out how to use the services provided by online dictionaries. Based on that you can decide the right tool/technology to be used.