This repo (abbreviated as "L10N" in this README) contains all translated/translatable strings for the fxa-content-server repo (abbreviated as "SOURCE"). It contains an exact copy of the locales/ directory from SOURCE, and nothing else (except for this README).
While the master branch will always follow the upstream version coming from the maintainer of the project, the webplatform-customizations branch will contain our own text differences.
On a regular basis, somebody on the SOURCE committers team copies everything from L10N into SOURCE/locale/, reviews the changes, then commits and pushes SOURCE. This makes the new translated strings available to SOURCE users, provides a clean (single-commit) revision history, and prevents non-locale/ changes from leaking from one repo to the other. The SOURCE repo contains a script (scripts/import_locale) to perform this update.
On a (different) regular basis, a SOURCE committer will update in the other direction. This starts with running the SOURCE repo's grunt "l10n-extract" task, which updates the *.pot files in SOURCE. The committer then copies the *.pot files from SOURCE into a checkout of L10N. Then they run ./scripts/merge_po.sh in L10N, which updates the *.po files in L10N. Then they should commit and push to L10N. That will update the set of translatable strings for the l10n team to work on.