Wath is the right branch of your repo to be used as base of my work?
Hmm, it really depends on what versions you'd like to support, the i18n framework that we fully support will be released with 6.7 and knowing the amount of work that needs to be done (Kibana right now has ~10-11k of localizable labels and this number is constantly growing) I'd say it makes sense to target 6.7 and 7.0 at least (these branches will be created later today or tomorrow from 6.x and master respectively).
We usually develop on master and then backport to older branches resolving conflicts if any, usually it goes flawlessly, but i18n is a different and totally new beast for us as well 