Prestashop 1.5.4, translation files are reported missing?

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I just upgraded our eCommerce site on our test server to PrestaShop 1.5.4 yesterday, and when viewing one of the shops, I get PHP errors stating that the translation .php files are missing for EN:

 

PHP Warning: include(/.../translations/en/errors.php) [<a href='function.include'>function.include</a>]: failed to open stream: No such file or directory in /.../admin#####/init.php on line 52, referer: http://our.site.com/admin#####/index.php?controller=AdminShopUrl&token=cee...5c0&id_shop=21

 

I tried downloading Prestashop to see if I can spot the difference, and noticed that there are just a bunch of .gzip files for BR, DE, ES, FR, IT, PL, RU. I suppose EN isn't here, as it's the default language, but nonetheless, why am I getting the above?

 

How do I fix it?