Clarify USE_I18N settings

@jzoldak showed me that setting `USE_I18N = DEBUG` didn't actually accomplish
what I had hoped it would -- changing DEBUG didn't also change USE_I18N.
This pull request accomplishes what I was trying to accomplish, without being
quite so clever about it.
Updated comment
Merge pull request #1269 from edx/db/clarify-debug

Clarify USE_I18N settings