Build: Move i18n_gspreadsheet to init task

Move to init so it isn't run on every build. This also allows local JSON
files to be copied to "site/data/i18n" for those behind proxies where
the task will not run.
i18n: Add back translation files

Files remain ignored so they won't be changed unless force added
Merge pull request #4721 from nschonni/v4-move-i18n-gs-to-init

Build: Move i18n_gspreadsheet to init task