Hi James,
May I know which language constant specifically is missing?
You can download the Twitter app from this page and the language file should be included(http://take.ms/wOmAD): https://stackideas.com/apps/item/3342-twitter-for-easysocial
This file will be located in JoomlaFolder/administrator/language/en-GB/en-GB.plg_app_user_twitter.ini

All of the admin language.

Hey James,
If reinstalling the app doesn't work. Can you provide me a screenshot of the page experiencing this on your site so I can check this? Thanks.

It's on an install on my localhost server. It's in the backend of the app itself.

Hey James,
Can you double check for me whether the Twitter app language file is in your backend file location: JoomlaFolder/administrator/language/en-GB/en-GB.plg_app_user_twitter.ini
If the file is there, can you upload that file here so I can check why the translations are not working inside that file. This is how the app page in backend looks like without the language file(http://take.ms/ZUetL).

That is correct, the file is not in the administrator's language folder.

I just manually uploaded the language file and all is fine. But something must be wrong with the package if the installer isn't distributing the language file itself.

Thanks for reported, I've checked again from those Facebook and Twitter installer xml file, it seems like we didn't include those language file, I will log this into our issue tracker.