Sandstorm.io recently added i18n, but only has a handful of community contributed translations so far: https://github.com/sandstorm-io/sandstorm/tree/master/shell/...
Sandstorm is a self-hostable web app platform which can help free people from proprietary cloud services. It's also a nifty way for people to develop and deploy small/niche web apps that aren't sustainable as a business on their own.
Various apps I use inside Sandstorm, of course, also could always use translation help as well. Etherpad, EtherCalc, Wekan, etc. Some are much more translated than Sandstorm, some are in about the same place. (EtherCalc could really use some translation love: https://github.com/audreyt/ethercalc/tree/master/l10n)
- https://github.com/homebrew/homebrew.github.io - https://github.com/ossfriday/ossfriday/ - https://github.com/github/opensource.guide/