Fetch IANA language subtag registry and format it
Summary:
Ref T1131
Test Plan:
Generate a database in the Darkbot format with:
./language-subtag-registry-datasource -l --format "lang+%%id%% %%fulldescription%%"
Reviewers: dereckson
Reviewed By: dereckson
Subscribers: fauve
Differential Revision: https://devcentral.nasqueron.org/D3156