This is a very tiresome to find out meaning of each unfamiliar word in a dictionary. Its draw your attentions away from reading and, furthermore, takes too much time.

Fortunately, there is a great solution: Google Translate service and several tools based on its functionality.

I think you're familiar with Google Translate. So, it's time to use features to translation on-the-fly.

You can use official Google Toolbar or FireFox plugin. (Google Chrome must be with pre-build functionality to translate on the fly, but I don't know it exactly, so I'll consider only Google Toolbar and Firefox plugin, thought it's not the only solution, but the most popular one, as for me.)

Google Toolbar
It's official and have rather rich functionality. We're interested in "on the fly" translation and Google Toolbar can do it. After some customization, translation will appear instantly right after you hover cursor over the word you want to translate.
Link: http://toolbar.google.com/

FireFox plugin
If you use FireFox (I hope so, 'cause it's the best browser ever - I use IE, Opera, Chrome, Safari etc. and FF is truly the best) you can use special plugin. There are several of them and I try most of them too. So, the best one is "Google Dictionary and Google Translate" plugin.
Link: https://addons.mozilla.org/en-US/firefox/addon/google-dictionary-and-goo...