I came across a very useful online dictionary today. It is bilingual and translates words between Arabic and English and also English and Arabic. Please find the URL below:
http://www.wordreference.com/aren/

A nice feature of this dictionary is that it provides a list of related words on a right hand side panel which you can click to learn new words.

It also presents the diacretics or voweling of the word so that you know how to pronounce it. Which can be very useful, e.g. I wrote the word (لغة) in the search box without the vowels, and this is the first thing in the result page.

Diacritics: لُغَة

In sum, this is a free online bilingual dictionary that provides the voweling of words in addition to a list of related (and sometimes unrelated) words that begin with the same letter. I think it is useful and I hope you find it so!