fall back to default locale in js i18n lookups

fixes CNVS-13232

test plan
- set your locale to something without translations
 (ask me how, it takes some console trickery)
- open the calendar
- ensure the calendar displays without error

Change-Id: Ic57167a6129ce7a881ce96798bcbcc8e5e0edc88
Reviewed-on: https://gerrit.instructure.com/35996
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Braden Anderson <braden@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Joel Hough <joel@instructure.com>