Whitelist latin script languages

Summary:
In https://www.wikidata.org/?diff=357938306, we can see tbe bot fills
labels in latin script for non latin scripts languages.

We're not sure this is what the community wants, and so plan to
restrict Dæghrefn to only fill labels for latin languages.

This whitelist is based on the latin script languages currently
available in l10n JSON files.

Test Plan: jsonlint l10n/latin-whitelist.json

Reviewers: Ash-Crow, dereckson

Tags: Dæghrefn

Differential Revision: https://devcentral.nasqueron.org/D519

Signed-off-by: Sébastien Santoro <dereckson@espace-win.org>