Directly run the prepare-multi-langs script after each site build.

Merged username-removed-25042 requested to merge pserwylo/fdroid-website:fix-77--i18n-script into master

Note that the workflow is still going to be a little sad, but that is related to the fact that it takes a lot of time to build a site in several languages. If that gets annoying, then I'd suggest reducing the number of languages in _config.yml -> languages during development.

Fixes #77 (closed).