It checks:
 - whether all keys used by the program are in the locale
 - whether the right number of %s are used
 - if there are no extra keys in the locale that shouldn't be there
This will become very useful when translations need to be updated for new MSE versions.

There is a perl script for generating the 'expected_locale_keys' resource file.
This file contains a list of all the locale keys used.
This is a resource and not a data file because it is automatically generated from the code,
 the user has no business modifying it.

I also fixed all the locale errors I found in the process.