Our on prem 2013 environment had issues loading:
/_layouts/15/plumsail/crosssitelookup/select2/select2_locale_en_us.js
On looking in the hive, the file was misnamed with a dash instead of underscore:
C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\15\TEMPLATE\LAYOUTS\Plumsail\CrossSiteLookup\select2\select2_locale_en-US.js
Thought you would like to know.