Zanata is a web-based translation platform that understands .properties files natively. It supports the standard .properties encoding (ISO-8859-1), but also non-standard utf-8 encoding since some choose to use it.
COI declaration: I am part of the core development team for Zanata.
If your project uses Maven, a Maven plugin is available to update source strings and translations as part of your build. Other clients are available for integrating with other build processes.
You can host open source projects on the public server (see http://zanata.org). For private development you can host your own server (see http://zanata.org/download/).
If you want more detail I'll update this answer.