You are using Calibre viewer, and your first language isn't Murican? 
You wanna look up a specific word while reading but, the Calibre default dictionary only translates from English to English?
Then, you are in luck, fellow readers! This is a tutorial about how to add the Google translate site as a dictionary of your choice!
- From the Calibre ebook viewer, go to preferences.
- Go to dictionaries then, click on 'add a website' as shown below

- A popup window will appear like this one>
5. Add the languages that your books are written in. i.e English, Arabic....etc (you can add as many as you like, they are not the languages that are going to be translated to. The link will do that)
7. Paste the URL as shown above, followed by {word} without spaces between them. It will be hidden at the end of this thread.
8. Change the language to yours in the URL
This is what you'll change. 'ar' refers to Arabic. i.e change it to 'es' if you want to translate from English to Spanish.
9. Save your changes. Now, all you have to do is highlight the word then right click, then click on 'Look up in dictionary [CRTL+L]'
P.S You can add any dictionary site besides the Google translate site with the same steps.
https://translate.google.com/?source=osdd#auto/ar/{word}
ShiroKan
13:30, 18th Nov 2018