To find out how many words are left to be translated in a language in a localization project which you own or administrate, go to the project page and access the Stats section.
Click on the Stats by languages tab and then on the language for which you want to check the wordcount.
Next to the language name, you can see how many words from the source language are translated (100/102 in our example) and how many words remain to be translated (2 untranslated words in our example).