BuddyPress.org

You could try GlotPress:
http://trac.glotpress.com

http://blog.glotpress.com – GlotPress blog

It’s not a plugin, but it allows you to translate scripts using a front-end interface.

There’s not much documentation or anything like that since it’s still quite an early script, but if you feel the need to edit language files using a front-end script, GlotPress is an option.

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[EDIT]
Just found the following WordPress plugin:
https://wordpress.org/extend/plugins/easy-translator/

It’s a little old, but you could give that a shot as well.