Today I was launching a new VPS to run my cloud instance of BTSync, and it happened that every new instance I launched gave me the same error, on both Ubuntu and Fedora.
This is the error I was getting on Ubuntu:
perl: warning: Setting locale failed. perl: warning: Please check that your locale settings: LANGUAGE = "en_US:", LC_ALL = (unset), LC_CTYPE = "UTF-8", LANG = "en_US"
This is the error from Fedora:
Failed to set locale, defaulting to C
It turns out, the problem is that your terminal is sending the locales from your local machine to your remote server, and although I don't have nor know the techincal details, I do have the solution:
nano /etc/ssh/sshd_config #AcceptEnv LANG LC_*
Once you do that, reboot your server (or the ssh daemon) and that should do it.
2015 10 01:
Read Really fix locale errors on an ssh connection from OSX for a better answer