You can add a dictionary/spell checker for firefox for a lot of languages. When you type a word in an textarea like this one and it's wrong it'll underline that word. With right click on it gives you suggestions for the correct word. It's really helpful especially when you have problems remembering how to write words (like I do).
I have it for German and English (you can even choose which country, US, British or Australian English)
Link: https://addons.mozilla.org/en-US/firefox/browse/type:3
You can also add a dictionary to your search engines. If you have one you often use just check back with firefox if that site is available as search engine.
http://mycroft.mozdev.org/search-engines.html
btw. if you dictionary can't translate a certain word because it's something very specific (a festival or food for example) you can also use wikipedia. Look for it in the original Language and then switch to the wiki in the languages you want to have the translation in.
Doesn't work every time but most of the times and it's especially helpful when your dictionary gives you 5 answers and you still don't know which one to use.
