Fallback-friendly MWLocalizedString to replace NSLocalizedString

If we don't get a localization back through the normal NSLocalizedString
channel, this falls back to an explicit load from the English localization,
instead of just handing you back the message key.

Bug: 63546

Change-Id: I465d76b3c48e36c14b91c9582d30229f5b0d541c