fall back to en on broken interpolation, fixes gh-485

fixes CNVS-14500

if interpolation keys are invalid for the current translation, fall back
to english

test plan:
1. run canvas with RAILS_LOAD_ALL_LOCALES=true
2. set your locale to spanish
3. go to the SIS imports page, e.g. http://canvas/accounts/1/sis_import
4. confirm that you don't get a page error
5. confirm that the page appears entirely in spanish, except the "Select
   the zip file that you want imported..." help text and link

Change-Id: I59aeba2abb133cc94d3156c8a39a04172f28b09b
Reviewed-on: https://gerrit.instructure.com/38453
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Matt Fairbourn <mfairbourn@instructure.com>