To find out the wordcount of words left to be translated in a language, access its page in your project and click on the Translation Forecast button in the menu on the right side of the page.
You'll see the estimated completion date of the translation, based on the translation pace, the number of new translations added to the language in the chosen interval, the number of translations left to be done and how many words the strings left to be translated sum up.
If you're interested in how many translations have been done in a language, how many words the translated strings represent or how many translations a contributor added, you can check the Stats section.