I think I got your proposal but sadly there is a problem this it since in the tests cases we are not using the real l10 but a mock. More concretely https://github.com/mozilla-b2g/gaia/blob/master/shared/test/unit/mocks/dialer/mock_lazy_l10n.js which as you will see does no real localization work :(

I'll send an update to this PR and we can continue discussing about it there if you need further information.