l10n: If both current and fallback strings are missing, EN fallback is used instead of EN current

[janbrasna]

Description

When both "current" and "fallback" strings are missing, the default locale value is not used from current string, but the fallback string.

Steps to reproduce

See e.g. https://www-dev.allizom.org/ca/account/ (catalá) having neither the current mozilla-account-sign-in-to nor the fallback firefox-accounts-sign-in-to translated.

The EN string is not used from mozilla-account-sign-in-to, but from the fallback firefox-accounts-sign-in-to — but if both are missing, the current EN should be used over EN fallback.

Expected result

"Sign in to your Mozilla account to:"

Actual result

"Sign in to your Firefox Account to:"